external: Use sub-Make for external projects.

Use sub-Make for external projects so they can reuse the make jobserver.

This removes the restriction to have only one job for some external projects. Instead now they should reuse the processes used for the main build.

(Note: These changes, with similar fixes in flesnet, reduced the compile time for flesnet in CbmRoot from ~2m30s to ~1m on my machine.)