This patch reverts the change to define _HAS_EXCEPTIONS=0 for Dinkumware
std library. It has the undesired effect of changing the ABI. It's better for
user code to handle the combination of turning off exceptions while treating
warnings as errors, and getting warnings/errors from the std library by having
the users silence the warning themselves.
This patch also fixes the case of asking to initialize any msvc toolset
versions (using msvc ;) when there are already versions initialized. Instead
of erroring to say that a version is already in use, it considers the set
of already initialized msvc toolsets as satisfying the request to generally
initialize msvc.