autotools: add support for 'unity' builds, enable in CI

Implement the "unity" builds as known from CMake, but for autotools.
It's limited to `lib` and `src` (CMake also supports it in `tests`).

Enable with: `--enable-unity` (disabled by default)

Unity builds speed up builds significantly. Cygwin and Windows builds in
particular, but the effect is noticeable on most systems. It also allows
discovering unity issues with autotools, benefitting also CMake when
building the same combination. In CI it makes turnaround times quicker.

This closes build performance with CMake. autotools still lags behind
because it builds shared and static libcurl in two, separate passes.
CMake does it in one. Manpage compilation isn't batched, it is in CMake.
After unity and test bundle support the slowest parts of the build are
the configuration phase (which is effectively a tedious, non-parallel,
compilation and/or linking of 300+ tiny programs. The next bottleneck
is compiling individual examples and finally test servers (only slow
with autotools).

The autotools implementation is slightly less efficient than CMake,
because 3 sources are permanently excluded while in CMake this isn't
necessary and solved more efficiently while building libtests. There is
also no 'unity' support for tests, making them a less efficient also.

Enable it in CI for most `configure` jobs. Except in GHA/dist (though
it works fine there too), to use the default config there. Also skip for
the Linux AWC-LC job where it made builds time a few seconds longer
(reason undiscovered.)

Autotools test suite builds compared between master -> `--enable-unity`:
- GHA/Linux: 32s -> 12s
  https://github.com/curl/curl/actions/runs/10705668823/job/29681617374
  https://github.com/curl/curl/actions/runs/10742978889/job/29796766297
- GHA/macOS: 37s -> 10s
  https://github.com/curl/curl/actions/runs/10705668813/job/29681632885
  https://github.com/curl/curl/actions/runs/10742978699/job/29796768875
- GHA/FreeBSD: 15m25 -> 10m58 (full workflow time, ~qemu)
  https://github.com/curl/curl/actions/runs/10705668811/job/29681607915
  https://github.com/curl/curl/actions/runs/10742978937/job/29796766115
- GHA/Cygwin: 3m32 -> 1m21
  https://github.com/curl/curl/actions/runs/10705668809/job/29681609965
  https://github.com/curl/curl/actions/runs/10742978645/job/29796756933
- GHA/MSYS2: 2m42 -> 50s
  https://github.com/curl/curl/actions/runs/10705668808/job/29681621166
  https://github.com/curl/curl/actions/runs/10742978662/job/29799739289
- GHA/mingw-w64: 5m32 -> 1m23
  https://github.com/curl/curl/actions/runs/10705668808/job/29681628787
  https://github.com/curl/curl/actions/runs/10742978662/job/29799741568

Closes #14815

.github/workflows/cygwin.yml

@@ -81,7 +81,7 @@ jobs:
         timeout-minutes: 5
         run: |
           PATH="/usr/bin:$(cygpath "${SYSTEMROOT}")/System32"
-          mkdir bld && cd bld && ../configure --enable-warnings --enable-werror \
+          mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
             --prefix="${HOME}"/install \
             --enable-websockets \
             --with-openssl \

.github/workflows/http3-linux.yml

@@ -451,7 +451,7 @@ jobs:
       - run: autoreconf -fi
         name: 'autoreconf'
 
-      - run: ./configure --disable-dependency-tracking ${{ matrix.build.configure }}
+      - run: ./configure --disable-dependency-tracking --enable-unity ${{ matrix.build.configure }}
         name: 'configure'
 
       - run: make V=1

.github/workflows/linux-old.yml

@@ -113,7 +113,9 @@ jobs:
         run: |
           mkdir bld-am
           cd bld-am
-          ../configure --disable-dependency-tracking --enable-warnings --enable-werror --with-openssl --enable-ares --with-libssh --with-zstd --with-gssapi --prefix="$PWD"/../install-am
+          ../configure --disable-dependency-tracking --enable-unity --enable-warnings --enable-werror \
+            --with-openssl --enable-ares --with-libssh --with-zstd --with-gssapi \
+            --prefix="$PWD"/../install-am
 
       - name: 'autoconf build'
         run: |

.github/workflows/linux.yml

@@ -496,7 +496,10 @@ jobs:
         if: ${{ matrix.build.configure }}
         name: 'autoreconf'
 
-      - run: ${{ matrix.build.configure-prefix }} ./configure --disable-dependency-tracking --enable-warnings --enable-werror ${{ matrix.build.configure }}
+      - run: |
+          ${{ matrix.build.configure-prefix }} \
+          ./configure --disable-dependency-tracking --enable-unity --enable-warnings --enable-werror \
+            ${{ matrix.build.configure }}
         if: ${{ matrix.build.configure }}
         name: 'configure (autotools)'
 
@@ -555,8 +558,7 @@ jobs:
       - if: contains(matrix.build.install_steps, 'pytest')
         # run for `tests/http` directory, so pytest does not pick up any other
         # packages we might have built here
-        run:
-          pytest -v tests/http
+        run: pytest -v tests/http
         name: 'run pytest'
         env:
           TFLAGS: "${{ matrix.build.tflags }}"

.github/workflows/linux32.yml

@@ -69,7 +69,9 @@ jobs:
       - run: autoreconf -fi
         name: 'autoreconf'
 
-      - run: ./configure --disable-dependency-tracking --enable-warnings --enable-werror ${{ matrix.build.configure }}
+      - run: |
+          ./configure --disable-dependency-tracking --enable-unity --enable-warnings --enable-werror \
+            ${{ matrix.build.configure }}
         name: 'configure'
 
       - run: make V=1

.github/workflows/macos.yml

@@ -203,7 +203,7 @@ jobs:
             options+=" --with-sysroot=$(xcrun --sdk macosx --show-sdk-path 2>/dev/null)"
             CFLAGS+=" --sysroot=$(xcrun --sdk macosx --show-sdk-path 2>/dev/null)"
           fi
-          mkdir bld && cd bld && ../configure --enable-warnings --enable-werror \
+          mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
             --disable-dependency-tracking \
             --with-libpsl=$(brew --prefix libpsl) \
             ${{ matrix.configure }} ${options}
@@ -605,7 +605,7 @@ jobs:
             [ '${{ matrix.config }}' = 'SecureTransport' ] && options+=' --with-secure-transport'
             CFLAGS+=' -mmacosx-version-min=${{ matrix.macos-version-min }}'
             # would pick up nghttp2, libidn2, but libssh2 is disabled by default
-            mkdir bld && cd bld && ../configure --enable-warnings --enable-werror \
+            mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
               --disable-dependency-tracking \
               --disable-docs --disable-manual \
               --without-nghttp2 --without-libidn2 \

.github/workflows/non-native.yml

@@ -136,7 +136,7 @@ jobs:
               pkgconf brotli openldap26-client libidn2 libnghttp2 nghttp2 stunnel py311-impacket
             autoreconf -fi
             export CC='${{ matrix.compiler }}'
-            mkdir bld && cd bld && ../configure --enable-debug --enable-warnings --enable-werror \
+            mkdir bld && cd bld && ../configure --enable-unity --enable-debug --enable-warnings --enable-werror \
               --prefix="${HOME}"/install \
               --enable-websockets \
               --with-openssl \
@@ -198,7 +198,7 @@ jobs:
           run: |
             ln -s /usr/bin/gcpp /usr/bin/cpp  # Some tests expect `cpp`, which is named `gcpp` in this env.
             autoreconf -fi
-            mkdir bld && cd bld && ../configure --enable-debug --enable-warnings --enable-werror \
+            mkdir bld && cd bld && ../configure --enable-unity --enable-debug --enable-warnings --enable-werror \
               --prefix="${HOME}"/install \
               --enable-websockets \
               --with-openssl \

.github/workflows/torture.yml

@@ -74,7 +74,9 @@ jobs:
       - run: autoreconf -fi
         name: 'autoreconf'
 
-      - run: ./configure --disable-dependency-tracking --enable-warnings --enable-werror ${{ matrix.build.configure }}
+      - run: |
+          ./configure --disable-dependency-tracking --enable-unity --enable-warnings --enable-werror \
+            ${{ matrix.build.configure }}
         name: 'configure'
 
       - run: make V=1

.github/workflows/windows.yml

@@ -103,7 +103,7 @@ jobs:
         if: ${{ matrix.build == 'autotools' }}
         timeout-minutes: 5
         run: |
-          mkdir bld && cd bld && ../configure --enable-warnings --enable-werror \
+          mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
             --prefix="${HOME}"/install \
             --enable-websockets \
             --with-openssl \
@@ -415,7 +415,7 @@ jobs:
       - name: 'autotools configure'
         if: ${{ matrix.build == 'autotools' }}
         run: |
-          mkdir bld && cd bld && ../configure --enable-warnings --enable-werror \
+          mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
             --host=${TRIPLET} \
             --with-schannel --with-winidn \
             --without-libpsl \

.github/workflows/wolfssl.yml

@@ -84,7 +84,7 @@ jobs:
       - run: autoreconf -fi
         name: 'autoreconf'
 
-      - run: ./configure --disable-dependency-tracking --enable-warnings --enable-werror ${{ matrix.build.configure }}
+      - run: ./configure --disable-dependency-tracking --enable-unity --enable-warnings --enable-werror ${{ matrix.build.configure }}
         name: 'configure'
 
       - run: make V=1