Global C++11 compatibility.

Our build system compiles flex/bison as C++ rather than C, but a few
projects add `-x c` to their flags, forcing the compiler to compile
them as C. This causes the compiler to reject the global C++ standard
flag, so we need to explicitly provide a C standard flag to override
it.

Bug: 18466763
Change-Id: Id68ad9317261ed4d857a949b07288bd137ff6303