Windows debug builds display dialog on abort() -- they should just abort

If a Windows debug build executable tries to abort by calling the C runtime function "abort()", the C runtime displays a dialog saying "Debug Error!" and "R6010 - abort() has been called". This screws up our tests, which are expecting abort() to exit the program, and causes the build slave to hang until the buildbot master kills it after 3 hours of no output.

We should tell the C runtime to just exit on abort and not display a dialog.