Fix pkgconfig metadata on windows

[h-vetinari]

Trying to fix abseil/abseil-cpp#1276 for conda-forge/grpc-cpp-feedstock#242 et al.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[h-vetinari]

@conda-forge/abseil-cpp @hmaarrfk This PR has the intention to be able to switch on a compilation test for grpc, which ends up needing correct pkgconfig metadata, which is a bit tricky for shared builds on windows.

There are two different levels here - shared builds result in only abseil_dll.dll & absl_flags_*.lib on windows, but packages consuming abseil are used to having way more targets (that - only on windows, of course - are all bunched into the abseil_dll for the shared build).

Hence the minimal effort to get correct metadata would be to add pkgconfig/abseil_dll.pc and abseil_flags_*.pc, but that would mean that any consuming package that's not taking care of this distinction would have to patch the pkgconfig metadata (example).

Therefore I think it'd be nicer if we just created the whole suite of absl_*.pc files, but point them to abseil_dll (avoiding the need to patch downstream), which is what this PR does. I've added tests for correctness of pkgconfig & CMake integrations, and am planning to try to upstream this after things work out also in e.g. grpc.

Thoughts?

[xhochy]

Sounds like a reasonable approach 👍

[h-vetinari]

Thanks @xhochy !

@hmaarrfk @isuruf
Any thoughts / objections?