There were two issues.
First is related on ISPC's CMake configuration forcing C and C++
compilers to be clang and clang++. This goes against of desired
behavior when we use our own compiled clang compilers.
The second issue was related on linker failure: CLang libraries
are linked statically, and they need some of C++ 11 STL symbols
which are coming from libstdc++.
Differential Revision: https://developer.blender.org/D8258
This requires ISPC for building OpenImageDenoise, so that is now added as
a dependency as well. Blender itself does not need ISPC for building so it
is not included as part of the precompiled libraries.
Differential Revision: https://developer.blender.org/D7641
