runtests.py: Also allow defining a minimum clang version in the C++XX extension setup.
runtests.py: Also allow defining a minimum clang version in the C++XX extension setup.

cython/cython,scoder/cython,scoder/cython,da-woods/cython,da-woods/cython,scoder/cython,da-woods/cython,da-woods/cython,scoder/cython,cython/cython,cython/cython,cython/cython