gcj: fix build on Darwin

Partially addresses https://github.com/NixOS/nixpkgs/issues/29715.

pkgs/build-support/setup-hooks/fix-darwin-dylib-names.sh

@@ -23,7 +23,14 @@ fixDarwinDylibNames() {
     for fn in "$@"; do
         if [ -L "$fn" ]; then continue; fi
         echo "$fn: fixing dylib"
-        install_name_tool -id "$fn" "${flags[@]}" "$fn"
+        int_out=$(install_name_tool -id "$fn" "${flags[@]}" "$fn" 2>&1)
+        result=$?
+        if [ "$result" -ne 0 ] &&
+            ! grep "shared library stub file and can't be changed" <<< "$out"
+        then
+            echo "$int_out" >&2
+            exit "$result"
+        fi
     done
 }
 

pkgs/development/compilers/gcc/6/default.nix

@@ -180,9 +180,14 @@ stdenv.mkDerivation ({
 
   hardeningDisable = [ "format" "pie" ];
 
+  prePatch =
+    (stdenv.lib.optionalString (langJava || langGo) ''
+      export lib=$out
+    '')
+
     # This should kill all the stdinc frameworks that gcc and friends like to
     # insert into default search paths.
-  prePatch = stdenv.lib.optionalString hostPlatform.isDarwin ''
+    + stdenv.lib.optionalString hostPlatform.isDarwin ''
       substituteInPlace gcc/config/darwin-c.c \
         --replace 'if (stdinc)' 'if (0)'
 
@@ -447,7 +452,7 @@ stdenv.mkDerivation ({
 
 // optionalAttrs (enableMultilib) { dontMoveLib64 = true; }
 
-// optionalAttrs (langJava) {
+// optionalAttrs (langJava && !stdenv.hostPlatform.isDarwin) {
      postFixup = ''
        target="$(echo "$out/libexec/gcc"/*/*/ecj*)"
        patchelf --set-rpath "$(patchelf --print-rpath "$target"):$out/lib" "$target"