Switch libpololu-avr port to GitHub sources.

The zip files released on the Pololu website confuse portscout
(which thinks there should be a new version every day!) and use a
non-standard directory hierarchy, which requires WRKSRC to be set.
Switching to the GitHub releases (which are created by the same company,
just using GitHub mechanisms) solves both of these problems.

Also, fix some problems pointed out by mat (thanks!) that I ought to have
caught earlier by running portlint -A. :(

Approved by: mat
Differential Revision: https://reviews.freebsd.org/D7879