trybot-testing: Use third_party clang on Mac. DO NOT COMMIT

build/standalone.gypi

Index: build/standalone.gypi
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 00c177819c06aca8ac792e5c6512916d0ea86100..f038aa836f3a48421e772880e14c65c2d7b255c1 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -730,6 +730,9 @@
         'SYMROOT': '<(DEPTH)/xcodebuild',
       },
       'target_defaults': {
+        'variables': {
+          'base_path%': '<!(cd <(DEPTH) && pwd -P)',  # Mac requires full path.
+        },
         'xcode_settings': {
           'ALWAYS_SEARCH_USER_PATHS': 'NO',
           'GCC_C_LANGUAGE_STANDARD': 'c99',         # -std=c99
@@ -768,8 +771,11 @@
           }],
           ['clang==1', {
             'xcode_settings': {
+              'CC': '<(base_path)/<(clang_dir)/bin/clang',
+              'LDPLUSPLUS': '<(base_path)/<(clang_dir)/bin/clang++',
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'CLANG_CXX_LANGUAGE_STANDARD': 'gnu++0x',  # -std=gnu++0x
+              'CLANG_CXX_LIBRARY': 'libc++'
             },
           }],
         ],