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>