Fix macro redefinition errors

This removes a race condition between both qwt and qt against math.h on
windows with _USE_MATH_DEFINES set. If math.h comes after qwt or qt
math headers the math defines will be redefined. Really nasty to track
down and ordering problems. So instead I just removed using math
defines on windows and fall back to asin definition of M_PI_2. This
will have some slight perf impact since it calls function instead of
using constant. But I doubt on any of today’s machine it really makes a
difference.

libs/mavlink/include/mavlink/v1.0/mavlink_conversions.h

 #ifndef  _MAVLINK_CONVERSIONS_H_
 #define  _MAVLINK_CONVERSIONS_H_
 
-/* enable math defines on Windows */
-#ifdef _MSC_VER
-#ifndef _USE_MATH_DEFINES
-#define _USE_MATH_DEFINES
-#endif
-#endif
 #include <math.h>
 
 #ifndef M_PI_2
-    #define M_PI_2 ((float)asin(1))
+    #define M_PI_2 ((float)asin(1.0f))
 #endif
 
 /**