gcc.rb: fix libgccjit linker flags

Library/Formula/gcc.rb

@@ -257,3 +257,23 @@ def caveats
 
  $(LIBGCCJIT_SONAME_SYMLINK): $(LIBGCCJIT_FILENAME)
 	ln -sf $(LIBGCCJIT_FILENAME) $(LIBGCCJIT_SONAME_SYMLINK)
+diff --git a/gcc/jit/jit-playback.c b/gcc/jit/jit-playback.c
+index 925fa86..01cfd4b 100644
+--- a/gcc/jit/jit-playback.c
++++ b/gcc/jit/jit-playback.c
+@@ -2416,6 +2416,15 @@ invoke_driver (const char *ctxt_progname,
+      time.  */
+   ADD_ARG ("-fno-use-linker-plugin");
+
++#if defined (DARWIN_X86) || defined (DARWIN_PPC)
++  /* OS X's linker defaults to treating undefined symbols as errors.
++     If the context has any imported functions or globals they will be
++     undefined until the .so is dynamically-linked into the process.
++     Ensure that the driver passes in "-undefined dynamic_lookup" to the
++     linker.  */
++  ADD_ARG ("-Wl,-undefined,dynamic_lookup");
++#endif
++
+   /* pex argv arrays are NULL-terminated.  */
+   argvec.safe_push (NULL);
+