mac ninja and make: Add support for GCC_ENABLE_OBJC_GC.

pylib/gyp/xcode_emulation.py

Index: pylib/gyp/xcode_emulation.py
===================================================================
--- pylib/gyp/xcode_emulation.py	(revision 1382)
+++ pylib/gyp/xcode_emulation.py	(working copy)
@@ -306,7 +306,6 @@
       self._WarnUnimplemented('COPY_PHASE_STRIP')
     self._WarnUnimplemented('GCC_DEBUGGING_SYMBOLS')
     self._WarnUnimplemented('GCC_ENABLE_OBJC_EXCEPTIONS')
-    self._WarnUnimplemented('GCC_ENABLE_OBJC_GC')
 
     # TODO: This is exported correctly, but assigning to it is not supported.
     self._WarnUnimplemented('MACH_O_TYPE')
@@ -378,10 +377,18 @@
     return cflags_cc
 
   def GetCflagsObjC(self, configname):
-    """Returns flags that need to be added to .m compilations."""
+    """Returns flags that need to be added to .m and .mm compilations."""
     self.configname = configname
+    cflags_objc = []
+
+    gc_policy = self._Settings().get('GCC_ENABLE_OBJC_GC', 'unsupported')
+    if gc_policy == 'supported':
+      cflags_objc.append('-fobjc-gc')
+    elif gc_policy == 'required':
+      cflags_objc.append('-fobjc-gc-only')
+
     self.configname = None
-    return []
+    return cflags_objc
 
   def GetCflagsObjCC(self, configname):
     """Returns flags that need to be added to .mm compilations."""