build verb: prevent concurrent invocation

[NikolausDemmel]

I just accidentally invoked catkin build twice in two shells on a large workspace. A long running build of an 126 package workspace was still running while I was continuing to work on something else and then wanted to rebuild just a part of the workspace. The High load warning of the job server tipped me off that the other build was still running (nice stuff!).

IMHO it would be a nice enhancement if catkin build would fail when already running on the workspace.

Overall IMO this is a simple, but also low-priority feature.

EDIT: We probably want to allow concurrent invocation of different profiles.

[jbohren]

Yeah this would be a good thing to add.