For conformance with general expectations, use M_PI, not PI.

(This is in global.h, not RageUtil.h, because we include math.h in global.h, and
M_PI is generally associated with math.h.  As some systems do provide it
in math.h, if we put it in RageUtil.h then we'll suffer breakage when people
on those systems use M_PI without including RageUtil.h.)

stepmania/src/global.h

@@ -145,6 +145,11 @@ typedef StdString::CStdString RString;
 
 /* Define a few functions if necessary */
 #include <cmath>
+
+#ifndef M_PI
+#define M_PI 3.1415926535897932384626433832795
+#endif
+
 #ifdef NEED_POWF
 inline float powf( float x, float y ) CONST_FUNCTION;
 float powf( float x, float y ) { return float( pow(double(x), double(y)) ); }