macopix: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

ld: dnd.o:src/main.h:136: multiple definition of
`MENU_EXT'; main.o:src/main.h:136: first defined here

pkgs/games/macopix/default.nix

···
 
   enableParallelBuilding = true;
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: dnd.o:src/main.h:136: multiple definition of
+  #     `MENU_EXT'; main.o:src/main.h:136: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   NIX_LDFLAGS = "-lX11";
 
   meta = {