Only add -isystem for libc++ for C++ files, not C files.

third_party/libc++/libc++.gyp

Index: third_party/libc++/libc++.gyp
diff --git a/third_party/libc++/libc++.gyp b/third_party/libc++/libc++.gyp
index 5a966fc96134453bd4040db0a00cf11617dee4c6..9bb3eeaf4b35e9e3bf5ae04165b69d342e255bf0 100644
--- a/third_party/libc++/libc++.gyp
+++ b/third_party/libc++/libc++.gyp
@@ -26,12 +26,10 @@
       'all_dependent_settings': {
         'target_conditions': [
           ['_type!="none"', {
-            'cflags': [
-              '-isystem<(libcxx_root)/trunk/include',
-              '-isystem<(libcxx_root)/../libc++abi/trunk/include',
-            ],
             'cflags_cc': [
               '-nostdinc++',
+              '-isystem<(libcxx_root)/trunk/include',
+              '-isystem<(libcxx_root)/../libc++abi/trunk/include',
             ],
             'ldflags': [
               '-stdlib=libc++',
@@ -88,12 +86,14 @@
         'trunk/src/valarray.cpp',
       ],
       'cflags': [
-        '-isystem<(libcxx_root)/trunk/include',
-        '-isystem<(libcxx_root)/../libc++abi/trunk/include',
         '-fPIC',
         '-fstrict-aliasing',
-        '-nostdinc++',
         '-pthread',
+      ],
+      'cflags_cc': [
+        '-nostdinc++',
+        '-isystem<(libcxx_root)/trunk/include',
+        '-isystem<(libcxx_root)/../libc++abi/trunk/include',
         '-std=c++11',
       ],
       'cflags_cc!': [