XFA: merge patch from CL 839163002 to only request C++ 11 for C++

XFA: merge patch from CL 839163002 to only request C++ 11 for C++

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
TBR=tsepez@chromium.org

Review URL: https://codereview.chromium.org/839163002

Committed: https://pdfium.googlesource.com/pdfium/+/0c753c3f98d089c184ca4fd6c11169e5796a1c48

[brucedawson, 2015-01-09 19:54:54]

Committed patchset #1 (id:1) manually as
0c753c3f98d089c184ca4fd6c11169e5796a1c48 (presubmit successful).

[Tom Sepez, 2015-01-09 20:05:52]

lgtm