manywarnings: add -fno-common
* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add -fno-common
to the list. Quoting the manual, "Compiling with -fno-common is
useful on targets for which it provides better performance, or if
you wish to verify that the program will work on other systems that
always treat uninitialized variable declarations this way [putting
it in the data section]." If diffutils had been using this sooner,
it would have prevented this duplicate declaration issue:
http://git.sv.gnu.org/cgit/diffutils.git/commit/?id=
v3.4-10-gc2dc91f