Make it build on llvm 3.5

Tried to take care to keep it compatible with <3.4.
clang++ 3.5 says:

`llvm_ops.cpp:547:52: error: use of undeclared identifier 'isfinite'; did you mean 'OpenImageIO::isfinite'?
OSL_SHADEOP int osl_isfinite_if (float f) { return isfinite (f); }
                                                   ^~~~~~~~
                                                   OpenImageIO::isfinite                                      `