Provide `OS::Mac.prerelease?` for pre-release checks and use it where
appropriate. This should simplify updating the test once a new OS X
release lands.
This also fixes a bug in `BuildError#dump`, where an empty warning
message was printed on El Capitan after a failed from-source build,
because the check there and the one in `check_for_unsupported_osx` were
out of sync.
ClosesHomebrew/homebrew#45257.
Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
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