Allow using the old std_cxx1x namespace.

This was supposed to work in a backward compatible way but I broke
it for everyone who didn't explicitly #included the old base/std_cxx1x/*h
headers but got stuff through indirect #includes.
Merge pull request #234 from bangerth/fix-std_cxx1x-vs-std_cxx11-compatibility

Allow using the old std_cxx1x namespace.