Fix building of GLSL demos which use M_PI

Some <math.h> files do not define M_PI, in which case, provide our own definition

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>

progs/glsl/twoside.c

@@ -16,6 +16,9 @@
 #include <GL/glut.h>
 #include "shaderutil.h"
 
+#ifndef M_PI
+#define M_PI 3.1415926535
+#endif
 
 static GLint WinWidth = 300, WinHeight = 300;
 static char *FragProgFile = NULL;