The current pkg-config setup has no sense of whether it is cross-compiling,
so it will detect things on the build system that are not present or are
wrong for the host system. This forces the cross-compiling build to only
look for pkg-config .pc files in --prefix.
A version of this has been the setup for many years with the Android builds.
Fixes#32191
Signed-off-by: Hans-Christoph Steiner <hans@eds.org>