garden-of-coloured-lights: add -fcommon workaround

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

ld: main.o:src/main.c:58: multiple definition of
`eclass'; eclass.o:src/eclass.c:21: first defined here

pkgs/games/garden-of-coloured-lights/default.nix

···
     sha256 = "1qsj4d7r22m5f9f5f6cyvam1y5q5pbqvy5058r7w0k4s48n77y6s";
   };
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: main.o:src/main.c:58: multiple definition of
+  #     `eclass'; eclass.o:src/eclass.c:21: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   meta = with lib; {
     description = "Old-school vertical shoot-em-up / bullet hell";
     homepage = "http://garden.sourceforge.net/drupal/";