Consider avoid use of non-standard cmake find methods

[Andrey Borzunov]

Consider avoid use of non-standard cmake find methods with fallback to vendored dependency in favour of find_package(PNG)

Expected Behavior

Cmake scripts should use only find_package. Provide dedicated cmake option to compile with vendored dependency.

Current Behavior

Non-standard find_libpng fails and silently build libpng with vendored zlib with wrong version, notwithstanding the fact, that we provided libpng 1.6.37 compiled with zlib 1.2.12 and compatible zlib 1.2.12.

Steps to Reproduce

Conan CI with recipe in PR

Build log

• 19.23:
• Github Release page: 19.23
• macOS, Clang (M1/arm64):
• Clang: 13.0/12.0:

[Davis E. King]

I'm not sure what you mean. dlib's cmake script uses find_package(PNG QUIET) to find libpng. Only if that fails does the fallback happen.

[Andrey Borzunov]

Actually, it uses include(cmake_utils/find_libpng.cmake) to find libpng here:

if (DLIB_PNG_SUPPORT)
         include(cmake_utils/find_libpng.cmake)
         if (PNG_FOUND)
            include_directories(${PNG_INCLUDE_DIR})
            set (dlib_needed_libraries ${dlib_needed_libraries} ${PNG_LIBRARIES})
         else()
            # If we can't find libpng then statically compile it in.
            include_directories(external/libpng external/zlib)
            set(source_files ${source_files}

Apparently, it works on default combination of OS/compiler/... but fails on non-default, whereas package manager provide dependencies that can be found using FindLibpng.cmake script in build directory using CMake find_package method.

[Adrià Arrufat]

I also don't get what you mean: include(cmake_utils/find_libpng.cmake) uses the “standard” find_package(PNG QUIET)

[Andrey Borzunov]

Sorry, my fault. Didn't examine cmake_utils/find_libpng.cmake. Indeed, issue is senseless in this wording.

Thanks.