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
78d48343ae