LP#1791184: ensure that 'make clean' doesn't delete pingest.pl

LP#1791184: ensure that 'make clean' doesn't delete pingest.pl

This patch corrects an issue where pingest.pl was treated as if
it were generated from a pingest.pl.in source file during the configure
step, meaning that it got deleted by 'make clean'.  This patch also
adds a couple helpful comments.

To test
-------
[1] Apply the patch.
[2] From a git checkout, run the configure and make steps, followed
    by a 'make clean'. Verify that pingest.pl is not deleted from
    the source tree.

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>