Require C++11

* configure.ac: Use MM_AX_CXX_COMPILE_STDCXX_11 from mm-common to check for
compiler support for C++11 and use it (-std=c++11 for current versions of g++).
Among other reasons, this is because libsigc++ and glibmm now require C++11,
and gmmproc generates C++11 code.