coriander: add -fcommon workaround

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

    ld: subtitles.o:src/coriander.h:110: multiple definition of
      `main_window'; main.o:src/coriander.h:110: first defined here

pkgs/applications/video/coriander/default.nix

@@ -22,6 +22,11 @@ stdenv.mkDerivation rec {
     sha256 = "0l6hpfgy5r4yardilmdrggsnn1fbfww516sk5a90g1740cd435x5";
   };
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: subtitles.o:src/coriander.h:110: multiple definition of
+  #     `main_window'; main.o:src/coriander.h:110: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   preConfigure = ''
     cp ${automake}/share/automake-*/mkinstalldirs .
   '';