Man page: document `brew fetch --build-from-source`

Closes #21070.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
qt5: 5.1

Note - pixeltool.app and qhelpconverter.app don't exist any more
in 5.1, that's why the rmtree calls were removed.

Also add HOMEBREW_PREFIX to configure in order to support brew
installations that are not in /usr/local

Closes #21154.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
Fix failing 10.6 image-related tests.