Show C++11 extension warnings by default with Clang
In Clang, disable `override` in C++03 mode
Disable -Werror in try_compile

As compilers become more advanced in their static analysis, we might
inadvertently disable some C++11 features because a new compiler version
finds something unrelated to complain about in our minimal examples in
the cpp0xtest folder.

We would rather fix warnings manually than automatically ensure that
C++11 features never emit any warnings.
Merge branch 'issue_145' to fix #145