Improve detection of C++11 mode

Improve detection of C++11 mode

This replaces commit 329eae56 with a better solution. Indeed, while
__cpluplus is useless with g++ 4.[3-6] because its value is always 1,
these compilers define __GXX_EXPERIMENTAL_CXX0X__ when "-std c++0x" is used.

Therefore the code now relies on both macros to detect C++11 mode
instead of setting it when --enable-cxx11 is used.

Also, use pure c++ mode instead of gnu++ extensions on gcc