cmake: make the ExternalProject test work

By micromanaging the project dependency and its inclusion into the test
project. It feels like an awkward construct, but perhaps better than
nothing.

It's also fragile because it's a static build with no assistance from
the external project (curl in this case). Mitigated in test by disabling
all dependencies and some features.

Since there is no special core cmake logic to be tested here, in CI
the test is tested really. To keep CI jobs at minimum, only add 3 of
them, taking 42s in total. (All 6 would take 270s.)

Follow-up to e2a23d5d0d #17203

Closes #18208

.github/workflows/distcheck.yml

@@ -301,6 +301,9 @@ jobs:
         with:
           persist-credentials: false
 
+      - name: 'via ExternalProject'
+        if: ${{ !contains(matrix.image, 'linux') }}
+        run: ./tests/cmake/test.sh ExternalProject ${TESTOPTS}
       - name: 'via FetchContent'
         run: ./tests/cmake/test.sh FetchContent ${TESTOPTS} -DCURL_USE_OPENSSL=ON
       - name: 'via add_subdirectory'
@@ -308,6 +311,20 @@ jobs:
       - name: 'via find_package'
         run: ./tests/cmake/test.sh find_package ${TESTOPTS} -DCURL_USE_OPENSSL=ON
 
+      - name: 'via ExternalProject (old cmake)'
+        if: ${{ contains(matrix.image, 'linux') }}
+        run: |
+          export TEST_CMAKE_CONSUMER; TEST_CMAKE_CONSUMER="$(cat ~/old-cmake-path.txt)"
+          if [[ "${MATRIX_IMAGE}" = *'macos'* ]]; then
+            export CFLAGS='-arch arm64'
+            export TEST_CMAKE_FLAGS='-DCURL_USE_LIBPSL=OFF'  # auto-detection does not work with old-cmake
+          fi
+          if [[ "${MATRIX_IMAGE}" = *'windows'* ]]; then
+            export TEST_CMAKE_GENERATOR='MSYS Makefiles'
+            export TEST_CMAKE_FLAGS='-DCMAKE_C_COMPILER=x86_64-w64-mingw32-gcc -DOPENSSL_ROOT_DIR=C:/msys64/mingw64'
+          fi
+          ./tests/cmake/test.sh ExternalProject ${TESTOPTS}
+
       - name: 'via add_subdirectory OpenSSL (old cmake)'
         run: |
           export TEST_CMAKE_CONSUMER; TEST_CMAKE_CONSUMER="$(cat ~/old-cmake-path.txt)"