Add --enable-rpath option to add -Wl,-rpath to pkg-config files.

This allows installing into e.g. /usr/lib/ffmpeg and binaries linked
against ffmpeg will pick these .so file while binaries linked against
some fork or different version will pick the libraries in /usr/lib.
There will be still some issues for binaries that (indirectly) end up
depending on multiple variants, but for the simpler cases it should allow
different applications to use different (compatible) variants that
are installed at the same time.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>