cmake: fix building with CMAKE_FIND_PACKAGE_PREFER_CONFIG=ON

By forcing local Find module invocations into `MODULE` mode. To avoid
accidentally picking up such a dependency via CMake Config-based
detection, which does not provide the expected imported target and make
the build fail.

It also prepares the scripts to alllow CMake Config support in local
Find modules.

Also:
- add reproducer in CI.
- allow custom options in CMake integration test script.

Refs:
91e06fde1b
https://github.com/curl/curl/pull/20784
https://github.com/curl/curl/issues/20729

Closes #2262

.github/workflows/ci.yml

@@ -192,6 +192,13 @@ jobs:
         if: ${{ !contains(matrix.image, 'windows') }}  # MSYS2 wolfSSL package not built with the OpenSSL compatibility option
         run: ./tests/cmake/test.sh find_package -DCRYPTO_BACKEND=wolfSSL
 
+      - name: 'via find_package (PREFER_CONFIG=ON)'
+        if: ${{ contains(matrix.image, 'windows') }}
+        run: |
+          export TEST_CMAKE_FLAGS_PROVIDER='-DCMAKE_FIND_PACKAGE_PREFER_CONFIG=ON'
+          export TEST_CMAKE_FLAGS_CONSUMER="${TEST_CMAKE_FLAGS_PROVIDER}"
+          ./tests/cmake/test.sh find_package -DCRYPTO_BACKEND=mbedTLS
+
       - name: 'via ExternalProject (old cmake)'
         if: ${{ contains(matrix.image, 'ubuntu') }}
         run: |

CMakeLists.txt

@@ -439,7 +439,7 @@ if(CRYPTO_BACKEND STREQUAL "OpenSSL" OR NOT CRYPTO_BACKEND)
 endif()
 
 if(CRYPTO_BACKEND STREQUAL "wolfSSL" OR NOT CRYPTO_BACKEND)
-  find_package(WolfSSL ${_specific_crypto_requirement})
+  find_package(WolfSSL MODULE ${_specific_crypto_requirement})
   if(WOLFSSL_FOUND)
     if(WOLFSSL_VERSION VERSION_LESS 5.4.0)
       message(FATAL_ERROR "wolfSSL 5.4.0 or greater required")
@@ -456,7 +456,7 @@ if(CRYPTO_BACKEND STREQUAL "wolfSSL" OR NOT CRYPTO_BACKEND)
 endif()
 
 if(CRYPTO_BACKEND STREQUAL "Libgcrypt" OR NOT CRYPTO_BACKEND)
-  find_package(Libgcrypt ${_specific_crypto_requirement})
+  find_package(Libgcrypt MODULE ${_specific_crypto_requirement})
   if(LIBGCRYPT_FOUND)
     set(CRYPTO_BACKEND "Libgcrypt")
     set(CRYPTO_BACKEND_DEFINE "LIBSSH2_LIBGCRYPT")
@@ -465,7 +465,7 @@ if(CRYPTO_BACKEND STREQUAL "Libgcrypt" OR NOT CRYPTO_BACKEND)
 endif()
 
 if(CRYPTO_BACKEND STREQUAL "mbedTLS" OR NOT CRYPTO_BACKEND)
-  find_package(MbedTLS ${_specific_crypto_requirement})
+  find_package(MbedTLS MODULE ${_specific_crypto_requirement})
   if(MBEDTLS_FOUND)
     if(MBEDTLS_VERSION VERSION_LESS 3.1.0)
       message(FATAL_ERROR "mbedTLS 3.1.0 or greater required")

cmake/libssh2-config.in.cmake

@@ -18,13 +18,13 @@ set(_libssh2_libs "")
 if("@CRYPTO_BACKEND@" STREQUAL "OpenSSL")
   find_dependency(OpenSSL)
 elseif("@CRYPTO_BACKEND@" STREQUAL "wolfSSL")
-  find_dependency(WolfSSL)
+  find_dependency(WolfSSL MODULE)
   list(APPEND _libssh2_libs libssh2::wolfssl)
 elseif("@CRYPTO_BACKEND@" STREQUAL "Libgcrypt")
-  find_dependency(Libgcrypt)
+  find_dependency(Libgcrypt MODULE)
   list(APPEND _libssh2_libs libssh2::libgcrypt)
 elseif("@CRYPTO_BACKEND@" STREQUAL "mbedTLS")
-  find_dependency(MbedTLS)
+  find_dependency(MbedTLS MODULE)
   list(APPEND _libssh2_libs libssh2::mbedcrypto)
 endif()
 

tests/cmake/test.sh

@@ -74,13 +74,13 @@ if [ "${mode}" = 'all' ] || [ "${mode}" = 'find_package' ]; then
   bldp='bld-libssh2'
   prefix="${PWD}/${bldp}/_pkg"
   rm -rf "${bldp}"
-  "${cmake_provider}" -B "${bldp}" -S "${src}" -G "${gen}" ${cmake_opts} -DCMAKE_UNITY_BUILD=ON ${TEST_CMAKE_FLAGS:-} "$@" \
+  "${cmake_provider}" -B "${bldp}" -S "${src}" -G "${gen}" ${cmake_opts} -DCMAKE_UNITY_BUILD=ON ${TEST_CMAKE_FLAGS:-} ${TEST_CMAKE_FLAGS_PROVIDER:-} "$@" \
     -DCMAKE_INSTALL_PREFIX="${prefix}"
   "${cmake_provider}" --build "${bldp}" --verbose
   "${cmake_provider}" --install "${bldp}"
   bldc='bld-find_package'
   rm -rf "${bldc}"
-  "${cmake_consumer}" -B "${bldc}" -G "${gen}" ${TEST_CMAKE_FLAGS:-} \
+  "${cmake_consumer}" -B "${bldc}" -G "${gen}" ${TEST_CMAKE_FLAGS:-} ${TEST_CMAKE_FLAGS_CONSUMER:-} \
     -DTEST_INTEGRATION_MODE=find_package \
     -DCMAKE_PREFIX_PATH="${prefix}/lib/cmake/libssh2"
   "${cmake_consumer}" --build "${bldc}" --verbose