Add -pthread to avoid std::thread etc. issues
Add -pthread to avoid std::thread etc. issues

With PHSA on Ubuntu 16.04, this issue gets triggered:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67791

franz/pocl,pocl/pocl,franz/pocl,pocl/pocl,franz/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,Oblomov/pocl,pocl/pocl,franz/pocl,Oblomov/pocl,Oblomov/pocl,pocl/pocl,pocl/pocl