The `disableConcurrentBuilds` pipeline option tells Jenkins to force
subsequent builds _of the same job_ to wait in queue until the one
running has completed. This is sufficient when there is only one
branch/project in development at a time. In order to prevent multiple
projects from running simultaneously, we need to acquire a global lock;
all projects need to have this same option in order for it to be
effective.
Dustin
43d15bdbf8