Add depended on libraries and C++ runtime to link to in pkg-config file

[robUx4]

When linking statically with chromaprint the app needs to know which libraries to link with. It also needs the C++ runtime if the calling code doesn't have any C++.

[robUx4]

This is needed when the host app is coded in C and has no idea that chromaprint uses C++ internally.

[lalinsky]

@robUx4 Could you post the diff of the pkg-config config on all platforms? The change are kind of hard to review and the diffs would help a lot.

[sebastinas]

As the C++ library is only needed for static linking, this should be added only to Libs.private if at all.

[lalinsky]

Indeed and static linking is tricky for multiple reasons, it's much better to just include the libraries needed for the particular build in the project's build system.

[robUx4]

On a Linux build, Before:

Name: chromaprint
Description: Audio fingerprint library
URL: http://acoustid.org/chromaprint
Version: 1.4.2
Libs: -L/root/build/contrib/x86_64-linux-gnu/lib -lchromaprint
Cflags: -I/root/build/contrib/x86_64-linux-gnu/include

After:

Name: chromaprint
Description: Audio fingerprint library
URL: http://acoustid.org/chromaprint
Version: 1.4.2
Libs: -L/root/build/contrib/x86_64-linux-gnu/lib -lchromaprint -L/root/build/contrib/x86_64-linux-gnu/lib -lavcodec -pthread -lm -latomic -lz -lgsm -lmp3lame -lm -lopenjp2 -lm -lavutil -pthread -lm -latomic -lstdc++ -lm -lc
Cflags: -I/root/build/contrib/x86_64-linux-gnu/include

I agree it my be better in Libs.private. Things like -pthread also don't belong here. Other dependencies like zlib might be handled through Requires: but that's a bit more work.

The C++ runtime to use is needed because the library can be used with a C interface. The host app cannot guess what it really needs to link to. When used from C++ code it will pick its own runtime (and should match the one from the library).