cerbero: Make all pkgconfig files reliably relocatable
We already tried to do relocatable pkgconfig files by using --define-prefix in pkg-config, but that was error-prone since it relied on the user using the correct pkg-config. It was also broken for any pkgconfig files that weren't in [PREFIX]/dir/pkgconfig, such as gstreamer plugin pc files.
Using ${pcfiledir} fixes it.
Fixes gstreamer#1643 (closed)