Fix building with `-fno-common`

* GCC 10 will switch the default to `-fno-common`.
  https://gcc.gnu.org/PR85678

Bug: https://bugs.gentoo.org/705880
Signed-off-by: Matt Turner <mattst88@gmail.com>
3 jobs for fno-common-for-1.20 in 8 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #1861030
debian-testing

00:00:28

 
  Build And Test
passed #1861031
autotools-build-and-test

00:08:19

passed #1861032
meson-build-and-test

00:01:57