Fix build on gp2x. Joystick handling of cursor is moving way too fast on the small 320x240 screen, but I can't understnad why...

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/release@1053 416bcca6-2ee7-4201-b75f-2eb2f807beb1

graph.c

@@ -47,6 +47,10 @@
 #include "windows.h"
 #include "input.h"
 
+#ifndef M_PI
+	#define M_PI 3.1415926535897932385
+#endif
+
 // Generic pixel-drawing function.
 Func_pixel Pixel_figure;
 
@@ -2853,4 +2857,4 @@ void Redraw_grid(short x, short y, unsigned short w, unsigned short h)
     Vertical_grid_line(col, y, h);
     col+= Snap_width*Main_magnifier_factor;
   }
-}
+}