add M_PI definition for VC6 which has a broken math.h

gfx/cairo/cairo/src/cairoint.h

@@ -62,6 +62,10 @@
 #include <limits.h>
 #include <stdio.h>
 
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
+
 #include "cairo.h"
 #include "cairo-debug.h"
 #include <pixman.h>