klystrack: add -fcommon workaround

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

    ld: libengine_gui.a(gui_menu.o):(.bss+0x0): multiple definition of
      `menu_t'; objs.release/action.o:(.bss+0x20): first defined here

pkgs/applications/audio/klystrack/default.nix

@@ -27,6 +27,12 @@ stdenv.mkDerivation rec {
     })
   ];
 
+  # Workaround build failure on -fno-common toolchains:
+  #   ld: libengine_gui.a(gui_menu.o):(.bss+0x0): multiple definition of
+  #     `menu_t'; objs.release/action.o:(.bss+0x20): first defined here
+  # TODO: remove it for 1.7.7+ release as it was fixed upstream.
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   buildFlags = [ "PREFIX=${placeholder "out"}" "CFG=release" ];
 
   installPhase = ''