Enable support for the C++11 override keyword. (#54)

Adds cpp0xtest/override_keyword.cpp which is an example program using
the override keyword.

In CMakeLists.txt, override_keyword.cpp is attempted compiled, and on
success, TPIE_CPP_OVERRIDE_KEYWORD is defined.

In config.h.cmake, a #define macro is inserted to turn `override` into
the empty string when it is not supported by the compiler.
Use the override keyword throughout TPIE.
Treat errors as warnings when testing for c++11 features
Add return statement to override_keyword.cpp.

With -Werror a compiler could be tempted to give a warning about a
missing return statement in non-void function, but that is not the
warning we are testing for.
Merge branch 'override'. Closes #54.