* Add the MM_AX_CXX_COMPILE_STDCXX() m4 macro,
to help other *mm projects build with C++11 or C++14 compiler support.
This is based on AX_CXX_COMPILE_STDCXX() but doesn't need to
be copied into projects manually.
It is less specific than MM_AX_CXX_COMPILE_STDCXX_11.
(Murray Cumming)
