Fix initCL, make conditional based on CLEW
Fix initCL, make conditional based on CLEW

When CLEW is present, CL functions become macros which may resolve to null
function pointers. A previous change attempted to guard against CL-related
crashes, but introduced compiler warnings.

This change conditionally tests the function when CLEW is present.

Resolves: #400

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