Generate pkg-config metadata on windows, resp. abseil_dll

[h-vetinari]

I'd like to have correct pkg-config metadata also on windows. 🙃

Based on looking at AbseilHelpers.cmake, there's only one place where pkg-config metadata is being generated, and it doesn't seem to cover windows, and almost certainly doesn't cover abseil_dll (c.f. also AbseilDll.cmake).

Would be nice to have this. :)

[h-vetinari]

Also, considering that abseil_dll is a bit of an outlier compared to the shared builds on unix (and understandably not a lot of projects take care of this distinction), it might be worth considering to generate dummy absl_<xyz>.pc files even for the DLL build, that simply do Requires: abseil_dll.

[h-vetinari]

Wow, I was not aware that issues can get closed across organisations... 🤔