Make sure that the compiler is strict in QtCore

ICC defaults to "fast math" mode, which allows it to do non-compliant
operations that may or may not result in unexpected values. Generally, it's
ok, but apparently the code in qlocale_tools.cpp is too complex, so we're not
taking chances.

I can't rule out an issue in the code, though.

Task-number: QTBUG-36795
Change-Id: Ica5cb77fb3a65d22ae8ad22e13b4ba78f1b5dadf
Reviewed-by: Lars Knoll <lars.knoll@digia.com>