Message 304503 - Python tracker

With the PR, Python 3.7 will still requires the C clock() function to build on Unix, since time.process_time() uses it as the last fallback if all other functions failed.

Maybe we can require to have other functions used by time.process_time() (clock_gettime(CLOCK_PROF), getrusage(), times(), ...), but I consider that it can be done later. This change may impact the Python portability: compilation error when building Python, see bpo-22624.