Explicitly provide path to C built-in includes in libc++ builds.

third_party/libc++abi/libc++abi.gyp

Index: third_party/libc++abi/libc++abi.gyp
diff --git a/third_party/libc++abi/libc++abi.gyp b/third_party/libc++abi/libc++abi.gyp
index 6e56ff92c8a784dfc472f2c21a67f0ca8cc5ad39..0a1ab54d1afabb79890ab0d10e286ee7d169333f 100644
--- a/third_party/libc++abi/libc++abi.gyp
+++ b/third_party/libc++abi/libc++abi.gyp
@@ -32,7 +32,9 @@
       ],
       'include_dirs': [
         'trunk/include',
-        '../libc++/trunk/include'
+        '../libc++/trunk/include',
+        # TODO(earthdok): remove when http://crbug.com/337426 is fixed
+        '../llvm-build/Release+Asserts/lib/clang/3.5/include/'
       ],
       'cflags': [
         '-g', '-O3', '-fPIC',
@@ -66,6 +68,10 @@
             'ldflags': [
               '-L<(PRODUCT_DIR)/lib/',
             ],
+            'include_dirs': [
+              # TODO(earthdok): remove when http://crbug.com/337426 is fixed
+              '../llvm-build/Release+Asserts/lib/clang/3.5/include/'
+            ],
           }],
         ],
       },