manywarnings: enable nicer gcc warning messages

manywarnings: enable nicer gcc warning messages

With gcc 4.3.4, -Wdisabled-optimization is noisy unless you also
use -funit-at-a-time.

With gcc 4.4.7, there is no indication which warning option
triggered a particular message unless you also use
-fdiagnostics-show-option.

Both of these options are default in newer gcc (such as 4.8.1);
but including them in the list of possible warnings makes the
experience nicer on older platforms.

* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Older gcc requires
some -f options for optimal warnings.

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