qt, qt5: cleanup hacky, unused, misnamed options.

Closes #22321.
Closes #22332.
Optionally use git to keep brew etc versioned.

Still in alpha state.
Handles defaults and merging changes with new versions.
Enable by setting the HOMEBREW_GIT_ETC environment variable.

Closes #15751.
Closes #17713.
ledger: fix missing gmp dependency.

Closes #22352.
doxygen: fix sourceforge mirror URL.

Closes #22351.