Force Visual C++ math.h to define the M_ constants

That header defines those constants outside of the math.h include guard,
so if someone #define'd _USE_MATH_DEFINES and #include'd math.h later,
they would get VC++ warnings about constants like M_PI being redefined
with different values (ours have parentheses, VS2010's math.h don't).

Instead, let's define _USE_MATH_DEFINES ourselves so we get the M_*
macros from math.h. That way, we won't redefine them outselves.

Task-number: QTBUG-45935
Change-Id: Idf715b895bac4d56b4afffff13db2c98ba8345c2
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/kernel/qmath.h

@@ -40,8 +40,18 @@
 
 #include <QtCore/qglobal.h>
 
+#ifndef _USE_MATH_DEFINES
+#  define _USE_MATH_DEFINES
+#  define undef_USE_MATH_DEFINES
+#endif
+
 #include <cmath>
 
+#ifdef undef_USE_MATH_DEFINES
+#  undef _USE_MATH_DEFINES
+#  undef undef_USE_MATH_DEFINES
+#endif
+
 QT_BEGIN_NAMESPACE
 
 #define QT_SINE_TABLE_SIZE 256