ipe records correct prefix

make and make install in ipe should specify different IPEPREFIX settings.
HOMEBREW_PREFIX for the former, prefix for the latter. The reason is that the
prefix specified to make is recorded internally and used to lookup things like
ipelets and stylesheets. Since other formulae may provide those, the location
should be "global." The latter prefix is used only for the installation
location.
cgal gets option --with-ipelets

Allows cgal to optionally install ipelets that are included, but not compiled by
default. Also changes --imaging -> --with-imaging.
cgal: update 4.5 bottle.
ipe: add 7.1.5 bottle.