Use $MAKE if set, otherwise fall back to system specific make command

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

closes #1035

Updates the way we select which make command to use:

Uses Sys.getenv("MAKE") if set otherwise uses current logic of selecting system specific make command.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

@WardBrian, mind taking a quick glance at this when you have a chance? (Not particularly urgent if you're busy, since what we have currently should still work.)

[jgabry]

The failures, as mentioned in #1035, were due to install_cmdstan() needing make before we could check cmdstan_version(). For now, we'll just default to $MAKE if set and otherwise keep the current logic.