gmtp: add -fcommon workaround

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

    ld: gmtp-preferences.o:src/main.h:72: multiple definition of
      `scrolledwindowMain'; gmtp-about.o:src/main.h:72: first defined here

pkgs/applications/misc/gmtp/default.nix

@@ -18,6 +18,12 @@ stdenv.mkDerivation {
 
   enableParallelBuilding = true;
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: gmtp-preferences.o:src/main.h:72: multiple definition of
+  #     `scrolledwindowMain'; gmtp-about.o:src/main.h:72: first defined here
+  # TODO: can be removed when 1.4.0 is released.
+  #NIX_CFLAGS_COMPILE = "-fcommon";
+
   preFixup = ''
     gappsWrapperArgs+=(--add-flags "--datapath $out/share");
   '';