Remove few more errors in mac scons build.

base/base_lib.scons

 # Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 __doc__ = """
 Configuration for building base.lib / libbase.a.
 """
 
 Import('env')
 
@@ skipping 74 matching lines @@
 env_dmg_fp = env.Clone()
 if env_dmg_fp['PLATFORM'] == 'win32':
   env_dmg_fp.Append(
       CCFLAGS = [
           '/wd4018',
           '/wd4102',
           '/wd4244',
           '/wd4554',
       ],
   )
-elif env_dmg_fp['PLATFORM'] == 'posix':
-  env_dmg_fp['CXXFLAGS'].remove('-Wall')
-  env_dmg_fp['CXXFLAGS'].append('-Wno-write-strings')
+elif env_dmg_fp['PLATFORM'] in ('darwin', 'posix'):
+  for var in ['CCFLAGS', 'CXXFLAGS']:
+    if '-Wall' in env_dmg_fp[var]:
+      env_dmg_fp[var].remove('-Wall')
+      env_dmg_fp[var].append('-Wno-write-strings')
 
 input_files.extend([
     env_dmg_fp.Object('third_party/dmg_fp/dtoa.cc'),
     env_dmg_fp.Object('third_party/dmg_fp/g_fmt.cc'),
 ])
 
 if env['PLATFORM'] in ('posix', 'darwin'):
   # Remove files that still need to be ported from the input_files list.
   # TODO(port): delete files from this list as they get ported.
   to_be_ported_files = [
@@ skipping 98 matching lines @@
       'message_pump_glib.cc',
       'nss_init.cc',
       'process_posix.cc',
       'process_util_linux.cc',
       'sys_string_conversions_linux.cc',
       'test_file_util_linux.cc',
       'worker_pool.cc',
   ])
 
 env.ChromeStaticLibrary('base', input_files)