build: enforce use of GCC 4.8 / Clang 3.4 / XCode Clang 5.1
build: enforce use of GCC 4.8 / Clang 3.4 / XCode Clang 5.1

Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib