Add missing M_PI definition in wave.c for Win32 compilation.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@3420 92316355-f0b4-4df1-b90c-862c8a59935f

pipi/filter/wave.c

@@ -22,6 +22,9 @@
 #include <stdio.h>
 #include <string.h>
 #include <math.h>
+#ifndef M_PI
+#   define M_PI 3.14159265358979323846
+#endif
 
 #include "pipi.h"
 #include "pipi_internals.h"