egoboo: add -fcommon workaround

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

        ld: mad.o:(.bss+0x233800): multiple definition of `tile_dict'; camera.o:(.bss+0x140): first defined here

pkgs/games/egoboo/default.nix

@@ -45,6 +45,11 @@ stdenv.mkDerivation rec {
     '';
   */
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: mad.o:(.bss+0x233800): multiple definition of `tile_dict'; camera.o:(.bss+0x140): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   NIX_LDFLAGS = "-lm";
 
   meta = {