fuzz: use a variable to allow specifying python-config to use Eventually we should probably default this to just `python-config` and have the oss-fuzz build.sh script specify the sanpy python-config, but for now this lets us make progress. Differential Revision: https://phab.mercurial-scm.org/D7563

fuzz: suppress deprecated-register warnings in our compile These come from the Python.h headers still using the `register` keyword and our use of C++17. I think this will go away when we're using Python 3 for our fuzzing, but that can come later. Differential Revision: https://phab.mercurial-scm.org/D7562

fuzz: follow modern guidelines and use LIB_FUZZING_ENGINE This will make our lives easier in an upcoming change, but it's also how we're supposed to set things up anyway. Differential Revision: https://phab.mercurial-scm.org/D7561