pkgconf: add UAVIF_DLL for static linking with pkgconf

[Biswa96]

allows for the usage of `pkgconf --static --cflags libavif' to produce
the proper CFLAGS for static linking. Relies on pkgconf rather than pkg-config.

This is as similar as https://gitlab.com/AOMediaCodec/SVT-AV1/-/merge_requests/1911

@kmilos @1480c1 @mechakotik

[vrabaud]

Thx! If it's only for static linking, how about changing the code to:

if(BUILD_SHARED_LIBS)
    target_compile_definitions(avif_obj PRIVATE AVIF_DLL AVIF_BUILDING_SHARED_LIBS)
    set(AVIF_PKG_CONFIG_EXTRA_CFLAGS " -DAVIF_DLL")
    set(AVIF_PKG_CONFIG_EXTRA_CFLAGS_PRIVATE "")
else()
    set(AVIF_PKG_CONFIG_EXTRA_CFLAGS "")
    set(AVIF_PKG_CONFIG_EXTRA_CFLAGS_PRIVATE " -UAVIF_DLL")
endif()

[Biswa96]

If it's only for static linking, how about changing the code to:

Yeah, this is a fix for static linking but the Cflags.private is only used when -static option is passed to pkgconf. So, having Cflags.private in pkgconfig file for shared library would not cause any issue. The svt-av1 project also has same change.

This is required in msys2/mingw packages. The projects provides both shared and static library in one package.

[vrabaud]

Why even define a variable then? Just put -UAVIF_DLL right in libavif.pc.cmake no?

[Biswa96]

Just put -UAVIF_DLL right in libavif.pc.cmake no?

Certainly, it is indeed a possibility. My approach was based on the definition of the AVIF_PKG_CONFIG_EXTRA_CFLAGS variable. If you prefer, I can remove the newly created variable as per your request.

[vrabaud]

Yes please, remove it: there is a AVIF_PKG_CONFIG_EXTRA_CFLAGS because it can be overwritten when BUILD_SHARED_LIBS is on. But AVIF_PKG_CONFIG_EXTRA_CFLAGS_PRIVATE is only set once so we might as well set it for good in libavif.pc.cmake.

[kmilos]

Please leave the private variable in even when building shared. In MSYS2 we ship both static and shared libs, but only one .pc (from the shared build run).

[vrabaud]

@kmilos , so it's ok to only have it in libavif.pc.cmake then, no need for AVIF_PKG_CONFIG_EXTRA_CFLAGS_PRIVATE right?

[Biswa96]

I have added -UAVIF_DLL in libavif.pc.cmake.