ntroduce OPTION for devel/cmake to generate packages

The generator causes segfaults because it doesn't call pkg_init()
(see https://gitlab.kitware.com/cmake/cmake/issues/18031).
This feature was briefly enabled by default for 3.11.0, between
r467437 (added) and r467620 (removed), but causes problems with
stage-qa (PR 227372). Hide it behind an OPTION for people who want
it and don't worry about stage-qa.

The patch to CPack source will be submitted upstream once some
corner cases are ironed out.

Reported by: upstream
Approved by: tcberner
Differential Revision: https://reviews.freebsd.org/D15900