macvim: 7.4.909 -> 8.1.1517

Fix up the macvim package to build again, with the latest snapshot. The
patchfile has been recreated by manually reapplying all of the changes
from the old patchfile, and the other changes in here were figured out
by trial and error (such as the need to unset `LD`).

Also tweak the package to use python37 by default, and add an option to
go back to python27 if desired.

Disable Sparkle so the user isn't prompted to update a readonly package.