Fix build for gcc version 10
gcc 10 defaults to `-fno-common`, which results in a lot of "multiple
definition" errors when linking. As suggested by the gcc documentation,
we workaround the issue for now by setting the `-fcommon` flag:
https://gcc.gnu.org/gcc-10/porting_to.html#common
Signed-off-by: Fazlul Shahriar <fshahriar@gmail.com>