CMake: detect C++ library for pkgconf file

[kmilos]

One option of addressing #943 (comment)

@pszemus Please test

[pszemus]

@kmilos GraphicsMagick builds OK. The built libheif.pc looks like this:

prefix=/home/pszemus/projects/imageresizer-worker/libs
exec_prefix=${prefix}
libdir=${exec_prefix}/lib64
includedir=${prefix}/include

# Contrary to older versions of libheif (<= 1.14.2), the available-codec-variables are now all set to 'yes'
# as since 1.14.0, codecs can be dynamically loaded at runtime and it is not possible anymore to determine
# the available codecs at compile-time. You'll get an unknown codec error at runtime if you try to use an
# unavailable codec.
builtin_h265_decoder=yes
builtin_h265_encoder=yes
builtin_avif_decoder=yes
builtin_avif_encoder=yes

Name: libheif
Description: HEIF image codec.
URL: https://github.com/strukturag/libheif
Version: 1.17.0
Requires:
Requires.private: dav1d libsharpyuv
Libs: -L${libdir} -lheif
Libs.private: -lstdc++
Cflags: -I${includedir}

[farindk]

Great. Thank you.