cmConfigure: Don't disable any warnings

It used to be the case that we could not include c++ standard headers without getting warnings. As a workaround, we disabled the warnings in cmConfigure.hand always included this file first.

Is that still necessary for our currently supported compilers?

I can confirm that it is not necessary for vc9.

If this can be merged, then we can remove a whole lot of "IWYU pragma: keep" annotations.