build: clean AM_CFLAGS and AM_CPPFLAGS usage around

also set commont include dirs.

fPIC and DPIC are automatically detected and added
as required by libtool. We don't need to carry it around.

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>