Support pkgconfig dependencies

[dfandrich]

pkgconfig is used mostly for linking against C/C++ libraries. The trick is to detect which pkgconfig libraries are in use since I'm not aware of a simple dependency format that lists them all. The most straightforward way will probably be to look for autoconf references (in configure.ac and its include files) to the PKG_CHECK_MODULES macro, or FindPkgConfig in CMakeLists.txt. That will catch many such uses but not all, since more will be found inside home-grown macros since those standard ones came about somewhat late in their respective projects' histories.