build: use a local version of pkg-config when necessary (Darwin-only).

Automatically enabled when pkg-config detection fails, and can
be forced with the --enable-local-pkgconfig configure option.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6019 b64f7644-9d1e-0410-96f1-a4d463321fa5