Only disable gmp on older mac for nightly testing.

The newer mac environment should be able to support gmp.
Couple of futures using setter in the where clause.
Update future text.
Merge branch 'master' of github.com:chapel-lang/chapel into test
Merge pull request #338 from sungeunchoi/test

Add a couple of futures using setting in where clause.

These are similar to test/functions/bharshbarg/setterInWhere.future (which does the same for `proc this`), but these get a different error message.
Merge pull request #339 from thomasvandoren/update-common-for-old-mac

Only disable gmp on older mac for nightly testing.

The newer mac environment should be able to support gmp.
Switch from using 'var' to 'ref' for return intent

This changes all occurances of RET_VAR to RET_REF in the compiler. A
warning will be displayed to the user if they try and use 'var'.
Update module code to new ref return syntax

The majority of the changes were done via:

`gsed -E -i 's/((proc|iter) .* +)var/\1ref/' [files]`

with a few outliers changed by hand.
Update our tests to the new ref return syntax