Add pkg-config file

[jhert0]

This would make it easier for developers to use your library.

[andlabs]

I considered this, and asked some people, but they indicated that the pkg-config file should really be written by the people packaging libui for the distribution they maintain. I'm not sure where I saw this; I have to look it up again... Keep this open until then.

[ddevault]

Distro maintainer here. It's the downstream's (i.e. your) job to provide the pkg-config file, and the distro will patch it if necessary (it usually isn't).

[andy5995]

I submitted PR #545 for this issue.

[ddevault]

Nice!