x11-toolkits/fox16: Update to 1.6.58

Improve Makefile readability, introducing a separator beween options
blocks. Pet portlint(1), removing unnecessary multiple instances of
USES=xorg. Lightly stroke portclippy(1) to address some other Makefile
order issues. I still think unique options descriptions should head
their respective blocks and not be kept in a block of their own as it
give a quick overview of what the option does right in the block and
makes wholesale removal of an option a tiny bit easier. On the fence
about the benefits of <option>_IMPLIES coming at the beginning or end
of an options block, but keeping it as-is (at the end) to avoid
unnecessary churn.

Remove GNU_CONFIGURE_MANPREFIX after 9fd7ee1cdf6c. There is no
perceivable need to cherry-pick this update to quarterly, so this is
safe.