Use private fields in pkgconfig files

This patch does a few related things:

• use Requires.private in pkgconfig files to stop forcing downstream packages to link against everything upstream
• added pkgconfig as BuildRequires for RPMs
• bumped required version of pkg-config to 0.18.0

This is mainly to prevent overlinking of gstlal* package requirements into downstream libraries that link against them (e.g. preventing gstlal-ugly linking unnecessarily against gslcblas when linking against gstlal).