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.

tectronics/cusp-library,tectronics/cusp-library,vkirangoud/cusp-library,victordion/cusp-library,tectronics/cusp-library,victordion/cusp-library,tectronics/cusp-library,baiyunping333/cusp-library,gshashidhar125/cusp-library,vkirangoud/cusp-library,vkirangoud/cusp-library,victordion/cusp-library,baiyunping333/cusp-library,gshashidhar125/cusp-library,baiyunping333/cusp-library,gshashidhar125/cusp-library,victordion/cusp-library,gshashidhar125/cusp-library,baiyunping333/cusp-library,vkirangoud/cusp-library