cerbero: Redefine prefix value in pkgconfig files

People that use the official binaries on macOS and Windows keep reporting issues saying that the prefix value in the pkgconfig file points to a non-existent path, pointing to the build prefix on the machine that the .pkg or .msi was built on.

Change all those to explicitly be a static non-existent path that also contains a message telling users what to do if they see it in their compiler command lines.

This is not needed on Linux because we do not ship official binaries for that, and also because it might break someone's cross-compile workflow since people build packages themselves on Linux.

Fixes #371 (closed)