Fix make error on MacOS
Fix make error on MacOS

uname -o isn't supported on MacOS, however, -s and works on all
platforms that we support via ponyup.

Closes #3832

sgebbie/ponyc,dipinhora/ponyc,jemc/ponyc,sgebbie/ponyc,jemc/ponyc,dipinhora/ponyc,sgebbie/ponyc,sgebbie/ponyc,sgebbie/ponyc,ponylang/ponyc,ponylang/ponyc,ponylang/ponyc,jemc/ponyc,dipinhora/ponyc