--maintainer-mode already enables -Werror but it it incompatible with
out-of-tree builds. Calling configure with CFLAGS=-Werror causes it to
misdetect some functions like llrint() due to redefinition warnings. And
setting CFLAGS when invoking make risks losing compiler options like -g
-O2.
So add a --enable-werror configure option to allow cleanly using
-Werror.
Signed-off-by: Francois Gouget <fgouget@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>