Black Mesa has stopped working after SDK2007 'update'

Don’t know if this has happened to anyone else today, but Steam decided that SDK2007 needed updating, I let it, and now BM doesn’t start up and I just get this error message:

See here

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.