typespeed: add -fcommon workaround

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

ld: typespeed-typespeed.o:/build/typespeed-0.6.5/src/typespeed.h:69: multiple definition of
`opt'; typespeed-file.o:/build/typespeed-0.6.5/src/typespeed.h:69: first defined here

pkgs/games/typespeed/default.nix

···
 
   patches = [ ./typespeed-config-in-home.patch ];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: typespeed-typespeed.o:/build/typespeed-0.6.5/src/typespeed.h:69: multiple definition of
+  #     `opt'; typespeed-file.o:/build/typespeed-0.6.5/src/typespeed.h:69: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   configureFlags = [ "--datadir=\${out}/share/" ];
   makeFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];