gargoyle: add -fcommon workaround

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

ld: build/linux.release/alan3/Location.o:(.bss+0x0): multiple definition of
`logFile'; build/linux.release/alan3/act.o:(.bss+0x0): first defined here

pkgs/games/gargoyle/default.nix

···
   buildInputs = [ SDL SDL_mixer SDL_sound gtk2 ]
     ++ lib.optionals stdenv.isDarwin [ smpeg libvorbis ];
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: build/linux.release/alan3/Location.o:(.bss+0x0): multiple definition of
+  #     `logFile'; build/linux.release/alan3/act.o:(.bss+0x0): first defined here
+  # TODO: drop once updated to 2022.1 or later.
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   buildPhase = jamenv + "jam -j$NIX_BUILD_CORES";
 
   installPhase =