Create a pkg-config file

[jsha]

As part of our build process, we should produce a .pc file indicating where the library and includes are, and what other libraries must be linked with it.

Some documentation: https://people.freedesktop.org/~dbn/pkg-config-guide.html

[thesamesam]

#82 would really make life a lot easier for distributions interested in looking at mod_tls & rustls. For one, it'd solve .pc and optionally dynamic libraries for anyone who wants them.

Additionally, distributions are used to handling cargo-c projects and it's easier than trying to figure out what a project has done that's completely homebrew.

The purpose of the rustls-ffi project is to make it easy to use rustls and it'd serve that aim well.

[lu-zero]

I can redo the patchset if there is interest on it.

[cpu]

A cargo-c generated pkg-config file is available as of v0.12.1 🎉