defining M_PI to fix some compiler errors on OSX 10.8
defining M_PI to fix some compiler errors on OSX 10.8

fixes #44

nyotis/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,xldeveloper/OpenSubdiv,nyotis/OpenSubdiv,shengfuintel/OpenSubdiv,lookup1980/OpenSubdiv,xldeveloper/OpenSubdiv,jcowles/OpenSubdiv,poljere/OpenSubdiv,lookup1980/OpenSubdiv,poljere/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,shengfuintel/OpenSubdiv,lookup1980/OpenSubdiv,c64kernal/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,takahito-tejima/OpenSubdiv,anshuarya/OpenSubdiv,poljere/OpenSubdiv,nyotis/OpenSubdiv,c64kernal/OpenSubdiv,jcowles/OpenSubdiv,anshuarya/OpenSubdiv,takahito-tejima/OpenSubdiv,anshuarya/OpenSubdiv,takahito-tejima/OpenSubdiv,jcowles/OpenSubdiv,takahito-tejima/OpenSubdiv-exp,c64kernal/OpenSubdiv,xldeveloper/OpenSubdiv,shengfuintel/OpenSubdiv