Adding constexpr to the C++11 compiler test.

This should catch the "interesting" case where an Intel-12 compiler
uses an STL from GCC-4.6 with -std=c++0x.  The latter supports the
constexpr keyword while the former does not.
Adding C++11 regex configure test.
Adding C++11 test for override.
Add test for C++11 initializer lists.
Adding test for C++11 variadic templates.
Add test for C++11 <thread> support.
Adding configure test for C++11 <type_traits> support.
Ran bootstrap.