Don't try to use 'echo -e', it's annoyingly non-portable.

Test plan:
Added
   add_warning 'test'; add_warning 'test2'
to this script and ran make, and saw the output was
   -- WARNINGS:

   WARNING: test
   WARNING: test2
Use `which python2.7` rather than assuming it's in /usr/bin.

Also, print warning messages as we go along, not just at the end.

Test plan:
Fingers crossed