Add pkg-config support

[tfc]

This PR adds pkg-config support. For every activated extra header lib, it creates an additional pkg-config file.

This way users can run pkg-config on suffixed packages to find out if that feature exists:

$ pkg-config --list-all
rapidcheck_boost      rapidcheck_boost - boost headers for rapidcheck
rapidcheck_boost_test rapidcheck_boost_test - boost_test headers for rapidcheck
rapidcheck_doctest    rapidcheck_doctest - doctest headers for rapidcheck
rapidcheck            rapidcheck - C++ framework for property based testing inspired by QuickCheck and other similar frameworks
rapidcheck_catch      rapidcheck_catch - catch headers for rapidcheck
rapidcheck_gtest      rapidcheck_gtest - gmock headers for rapidcheck

$ pkg-config --exists rapidcheck_gtest && echo yes
yes

$ pkg-config --cflags rapidcheck_gtest
-I/nix/store/24ywzksid0yihlbbbk2yr0074zm06raa-rapidcheck-unstable-2023-04-16/include

[Ericson2314]

@emil-e it would be good to merge this because we want to use it in Nixpkgs and for Nix.

[Ericson2314]

Thanks!

[h0nzZik]

Is PKG_CONFIG_LIBS intentionally empty?

[Ericson2314]

Yes I think so, otherwise it won't be replaced or will be a warning/error.

[teh]

I'm not sure this is correct. rapidcheck ships a librapidcheck.so file which doesn't get picked up with --libs because this is empty

[octurion]

I think the correct value for PKG_CONFIG_LIBS should be "-L\${libdir} -lrapidcheck"

[Ericson2314]

Oh nevermind that's not correct. @octurion is right. Sorry!

[roberth]

#326 now intends to apply @octurion's suggestion.