Add 'make check' to the top level Makefile.

I know that you guys are still using poor old tar_em_up.sh directly, but I'm too lazy for that, I always just run make in the top level directory and be done with it. ;-)

One feature that was still missing in the Makefile was a convenience target to run the regression tests in the scripts folder. This MR takes care of that. Having finished make, you can now do make check1 to just run regression_tests.pd, or make check to run the full test suite including the external tests. Tested on Linux, Mac, and Windows, works everywhere.

Jonathan, I cancelled the CI since it doesn't test the top level Makefile anyway. So this is ready to go in right away. I'd appreciate that very much, since this little target makes my life easier, thank you!

BTW, with this MR the CI should now be easy to adapt to use the top level Makefile instead of running tar_em_up et al directly. No need to mess with BUILD_TYPE flags any more, that way the CI config should look quite a bit cleaner. I can prepare a merge request for that, too, if you want me to; just let me know.