Without the change the build against upstream gcc-10 build fails as:
ld: sprite.o:/build/0verkill/data.h:171: multiple definition of `obj_attr'; server.o:/build/0verkill/data.h:171: first defined here
ld: sprite.o:/build/0verkill/data.h:142: multiple definition of `weapon_name'; server.o:/build/0verkill/data.h:142: first defined here
Use -fcommon as a workeround until upstream updates the code.