Add pkg.m4 from pkg-config 0.28 to prevent more autogen.sh work
Use PKG_CHECK_MODULES() for calling and overriding pkg-config

- macro is in build/pkg.m4 from pkg-config 0.28
- Use <PREFIX>_CFLAGS and <PREFIX>_LIBS envars.
- Enables overriding of pkg-config if needed.
- Used for raptor 2 and libuuid

Fixes Issue #0000537
http://bugs.librdf.org/mantis/view.php?id=537