Remove ad-hoc M_PIs and enable VC++ macro

Fixes #564.

examples/gears.c

@@ -21,6 +21,10 @@
  *   - Enabled vsync
  */
 
+#if defined(_MSC_VER)
+ // Make MS math.h define M_PI
+ #define _USE_MATH_DEFINES
+#endif
 
 #include <math.h>
 #include <stdlib.h>
@@ -28,10 +32,6 @@
 #include <string.h>
 #include <GLFW/glfw3.h>
 
-#ifndef M_PI
-#define M_PI 3.141592654
-#endif
-
 /* If non-zero, the program exits after that many seconds
  */
 static int autoexit = 0;