build-sys: use -Wno-clobbered for some BSD crap

 * extend UL_WARN_ADD() for accept non-default CFLAGS variable
 * add BSD_WARN_CFLAGS with -Wno-clobbered
 * use all this for pg(1) and more(1)

Signed-off-by: Karel Zak <kzak@redhat.com>