devel/trellis: prepare for boost 1.89

The libboost_system component will no longer exist with the new boost
version and even now it's just a dummy. Don't let cmake look for it
to fix the build.

Trellis still fails on armv7 2025Q3, but that's due to a boost bug that
is already fixed in main.

Also add a cmake define to not have cmake be confused if git is present,
as it would otherwise try to find a git commit that isn't there.

Approved by: portmgr (build fix blanket)