Ok, I've resolved this, and the linker errors did actually point at the root issue.  They show trying to link extensions against /usr/local/lib/libpython2.7.a which was my test static build of 2.7.16.

Arguably this seems an issue in the buildbot build process in that it doesn't sandbox extension building to force linking to the python library being built rather than a random system library, but I can't see spending any time on that.  I've purged the 2.7.16 files from the /usr/local tree and things seem to be back to normal

I've started a new run for the failing build.