The former appended the necessary switch to enable C++11 to the CXXFLAGS
whereas AX_CXX_COMPILE_STDCXX appends it to CXX which has the benefit
that all C++ sources are compiled using the same C++ standard. Therefore
it is no longer necessary to manually hardcode '-std=c++11' anywhere
like we did in the Ruby bindings linker command and assures that the
compilation of them is done with C++11 support as well.
This fixes bug #795