Build problem on Mac with brew

[Scott Baden]

A recent change (last week) in brew's python installation results in the loss of python2

The fix is

install python2: breew install python@2

Aed it to your path

export PATH="/usr/local/opt/python@2/bin:$PATH"

[Dan Bonachea]

This was discussed in today's meeting John will be looking into a workaround for this.

ideally an UPCXX_PYTHON envvar to force a particular python

[Dan Bonachea]

Resolved in the 2018.3.0 release