Make VertexKernelBatch public when building with clang.
Make VertexKernelBatch public when building with clang.

There seems to be a bug in clang: despite the fact that FarMeshFactory is
declared to be a friend class from FarSubdivisionTables, clang protests that
VertexKernelBatch is not accessable.

This hack adds an #ifdef __clang__ block to make that structure public and keep
Far compiling under OSX.

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