manywarnings: cater to more gcc infelicities

manywarnings: cater to more gcc infelicities

Some versions of gcc (at least 4.2.1) noisily warn if
-Wuninitialized is given without -O; in isolation, configure
learns that the warning option is supported, but when later
compiling with -Werror, this makes builds fail.  If the
option will have no effect for the configured choice of
$CFLAGS, then it is not worth including in the set.

* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add test for
-Wuninitialized without -O.

Signed-off-by: Eric Blake <eblake@redhat.com>