Define _USE_MATH_DEFINES for Visual Studio (thanks @pionere!)

Fixes https://github.com/libsdl-org/SDL/issues/3790
(cherry-picked from commit c6e89619794ed6307f9efb0b242a2cdb605e1dc3)

include/SDL_stdinc.h

@@ -80,9 +80,9 @@
 # include <ctype.h>
 #endif
 #ifdef HAVE_MATH_H
-# if defined(__WINRT__)
+# if defined(_MSC_VER)
 /* Defining _USE_MATH_DEFINES is required to get M_PI to be defined on
-   WinRT.  See http://msdn.microsoft.com/en-us/library/4hwaceh6.aspx
+   Visual Studio.  See http://msdn.microsoft.com/en-us/library/4hwaceh6.aspx
    for more information.
 */
 #  define _USE_MATH_DEFINES