Set -fcommon compile option
In gcc 10, the default was changed to -fno-common, triggering linker
errors since the code indeed uses extern declarations in such a way.
Turns out fixing these isn't easy, so rather just restore the previous
behaviour.
- GnuPG-bug-id: T5215
- Signed-off-by: Christoph Biedl <cb@gnupg.org>