Work around pkg-config not being used on non-FreeBSD systems.

This is a horrible hack but I can't seem to find a better way of
fixing it which won't end up being the entire build system rewrite
which is happening in the master branch.
Add a work around for trampling over PKG_CONFIG_PATH.

The extend_pkg_path method will be removed in 2.2 as it
causes problems with non-standard pkg-config paths. This
commit adds a workaround until that happens.