Added M_PI work-around to cusp/gallery/diffusion.h.
Added M_PI work-around to cusp/gallery/diffusion.h.

It would be better to have a consistent fix to this problem, which also
occurs in cusp/complex.h.  This fix will fail if <math.h> or <cmath> is
included first somewhere else.

cusplibrary/cusplibrary,sdalton1/cusplibrary,cusplibrary/cusplibrary,sdalton1/cusplibrary,cusplibrary/cusplibrary,cusplibrary/cusplibrary,sdalton1/cusplibrary,sdalton1/cusplibrary