Makefile: Add -fno-common to compiler flags
Makefile: Add -fno-common to compiler flags

GCC 10 will default[0] to using -fno-common[1]. This looks like a useful
thing to do now...

[0]: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fbf14af879a84acfc8519caea0b30f2e367634de
[1]: https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fcommon

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac