As suggested by Max Horn.

This exists since gap 4.10 and will only run the install checks once
while also exiting with an appropriate exit code. This new check has
uncovered some test failures, which are harmless and actually disabled
in a future gap release. The error-detection code for the previous test
target was probably broken.

GAP downloads moved to a new subdomain. The old one still redirects, but
the new one is the "source of truth" so we should switch to that.

No longer necessary, since the build system was significantly overhauled
/ replaced by Max Horn.

Technically optional but strongly recommended from upstream. Replaces
their own implementation for input parsing.

System gmp is now the default.

gap: cleanup