Attempt to work with Visual Studio in -Za (strict ANSI) mode

Visual Studio always treats enum values as signed int, even when the
value doesn't fit in a signed int (like 0x80000000 or larger than 32-
the tags themselves are still signed. That causes ambiguity in creating
a QFlag from an enum value.

Visual C++ defines __STDC__ in C mode, but we have no macro in C++ mode.

Also note that the Windows SDK headers don't compile in -Za mode.

Task-number: QTBUG-39700
Change-Id: Ia943cef37ac1f539bd461c3c18200b0c365c72b3
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>