Enable the SuiteSparse:GraphBLAS JIT

[eriknw]

This uses C config from sysconfig by default to try to make the JIT "just work" most of the time (as long as Python knows about a C compiler).

@michelp, I think you have explored using the JIT as much as anybody, so I would appreciate it if you took a careful look at this PR. Did I set everything correctly using sysconfig? I trust you more than myself for this ;)

Open questions:

• Can we easily detect whether a C compiler exists (w/o shelling out); if so, should we disable if it doesn't?
• What about OpenMP, which is very important for performance? Should we add or check for anything?
• Does anybody want to add tests? :-P

[alugowski]

I think tests are a must. There appear to be quite a lot of settings to set with lots of opportunities for things to go wrong.