configure: if no perl, disable unity and shell completion, related tidy ups

Also:
- GHA/linux, macos: test `install` with autotools too.
- GHA/linux, macos: enable fish and zsh completion in an autotools job.
- scripts: drop dynamic perl checks redundant after this patch.
- scripts: drop two interim variables.
- autotools: make `build-certs` target check perl first.
- autotools: replace `$(PERL)` with `@PERL@` to match the pattern used
  in most automake scripts. For consistency. This makes `PERL` be
  defined at configure-time, as opposed to make-time, for these cases
  now.

Closes #18141

.github/workflows/macos.yml

@@ -238,7 +238,7 @@ jobs:
           - name: 'OpenSSL libssh c-ares'
             compiler: clang
             install: libssh
-            configure: --enable-debug --with-libssh --with-openssl=/opt/homebrew/opt/openssl --enable-ares
+            configure: --enable-debug --with-libssh --with-openssl=/opt/homebrew/opt/openssl --enable-ares --with-fish-functions-dir --with-zsh-functions-dir
           - name: 'OpenSSL libssh'
             compiler: llvm@18
             install: libssh libnghttp3
@@ -423,7 +423,7 @@ jobs:
             fi
             [ -n "${MATRIX_MACOS_VERSION_MIN}" ] && CFLAGS+=" -mmacosx-version-min=${MATRIX_MACOS_VERSION_MIN}"
             [[ "${MATRIX_INSTALL_STEPS}" = *'pytest'* ]] && options+=' --with-test-vsftpd=no'  # Skip ~20 tests that stretch run time by 7x on macOS
-            mkdir bld && cd bld && ../configure --enable-unity --enable-warnings --enable-werror \
+            mkdir bld && cd bld && ../configure --prefix="$PWD"/curl-install --enable-unity --enable-warnings --enable-werror \
               --disable-dependency-tracking \
               --with-libpsl=/opt/homebrew/opt/libpsl \
               ${MATRIX_CONFIGURE} ${options}
@@ -452,9 +452,13 @@ jobs:
       - name: 'curl version'
         run: bld/src/curl --disable --version
 
-      - name: 'cmake install'
-        if: ${{ matrix.build.generate }}
-        run: cmake --install bld --strip
+      - name: 'curl install'
+        run: |
+          if [ "${MATRIX_BUILD}" = 'cmake' ]; then
+            cmake --install bld --strip
+          else
+            make -C bld V=1 install
+          fi
 
       - name: 'build tests'
         run: |