Use lessc for building media, if available.

Our media building uses our bless service for compiling .less files to
.css. However, this isn't suitable for packaging on RHEL/Fedora/Debian.

We now check to see if lessc is available on the system, and use that
instead of bless. The packagers can then provide lessc as a build
dependency. It also makes for faster building of media.

Tested building with and without lessc. Verified that the appropriate
pipeline compiler was being used.