Prevent leaking _USE_MATH_DEFINES outside of qwt_math.h

[Goran Z. <GZ...@ti...>]

Hi there!

In qwt_math.h there is this piece of code:

#if defined(_MSC_VER)
/*
  Microsoft says:

  Define _USE_MATH_DEFINES before including math.h to expose these macro
  definitions for common math constants.  These are placed under an #ifdef
  since these commonly-defined names are not part of the C/C++ standards.
*/
#define _USE_MATH_DEFINES 1
#endif

#include <qmath.h>
#include "qwt_global.h"


I believe one should also #undef _USE_MATH_DEFINES after qmath.h has been
included so Visual Studio would not generate warnings about macro redefinitions:
C:\Program Files (x86)\Windows
Kits\10\Include\10.0.17763.0\ucrt\corecrt_math_defines.h:17: warning: C4005:
'M_E': macro redefinition
C:\Qt\5.14.2\msvc2017_64\include\QtCore\qmath.h:155: see previous definition of
'M_E'


See qmath.h how it is done.

I proposed following fix to be upstreamed:
// Prevent _USE_MATH_DEFINES to leak outside of this header file.
#if defined(_MSC_VER)
#undef _USE_MATH_DEFINES
#endif


/Goran