These macros come from systemd, but CC_CHECK_CFLAGS_SILENT was removed in
systemd commit eb2e280f9c59b66965c9316eadc4c113a13ca744, breaking some of
them.

CC_FLAG_VISIBILITY doesn't need to require CC_CHECK_WERROR because
CC_CHECK_FLAG_APPEND always append -Werror anyway. Which kinda brings into
question why we have CC_CHECK_WERROR in the first place, but whavever.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>