CMake pkg-config modulification

[tiennou]

This is the CMake module I wrote for mbedTLS, which tries to streamline writing pkg-config files, in an attempt to fix #5203.

This is incomplete (I think), but I'll need guidance, because macOS usually doesn't really care about *.pc files anyways.

Here's a gist of the current changes introduced, at least on macOS. I do think the Libs.private looks "saner", but the Requires.private isn't there yet.

[pks-t]

I like this quite a lot. As you said, Requires.private is still missing, but apart from that it already looks quite nice to me. Does this also handle paths with spaces correctly, e.g. for the prefix or linker flags?

[tiennou]

Okay, I've added some quoting in the hope that would be sufficient 😉. I've updated the gist with an OpenSSL, non-PCRE run, as a syntax check.

[pks-t]

Tried it on my own system and everything looked good to me. Thanks a lot, @tiennou!