PKG_CHECK_MODULES requires both _CFLAGS and _LIBS to be set to skip the
check.

Change-Id: Ia4ae0576f186eb0c002c40bdb66f3a11a972a0c3
(cherry picked from commit 969255f0)
Reviewed-on: https://gerrit.libreoffice.org/16505Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>