darwin.developer_cmds: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
clang-11. Otherwise build fails as:

    duplicate symbol '_btype_2' in:args.o pr_comment.o

pkgs/os-specific/darwin/apple-source-releases/developer_cmds/default.nix

@@ -16,6 +16,10 @@ appleDerivation {
       --replace "/usr/bin/cpp" "$out/bin/clang-cpp"
   '';
 
+  # Workaround build failure on -fno-common toolchains:
+  #   duplicate symbol '_btype_2' in:args.o pr_comment.o
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   # temporary install phase until xcodebuild has "install" support
   installPhase = ''
     for f in Products/Release/*; do