Only request C++11 when compiling C++ code.

Linux pdfium builds have 215 warnings of this form:
command line option -std=gnu++11 is valid for C++/ObjC++ but not for C

The obvious fix is to not request C++11 for C compilations. The only
complication was the the switch we are using is actually gnu++0x, not
gnu++11.

BUG=https://code.google.com/p/pdfium/issues/detail?id=102
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/839163002
diff --git a/build/standalone.gypi b/build/standalone.gypi
index 0d650a3..645b9f2 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -150,9 +150,9 @@
       '-pthread',
       '-fno-exceptions',
       '-fvisibility=hidden',
-      '-std=gnu++0x',
     ],
     'cflags_cc': [
+      '-std=gnu++0x',
       '-Wnon-virtual-dtor',
       '-fno-rtti',
     ],