msvc2015: Align compiler flags with rest of Qt build

msvc-desktop.conf does disable the exception warning for building all
modules, so use the same set of compiler flags for building qmake.

Change-Id: I97026f3cb78e656e8de76e1c8afe19cec6501499
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>