gcc parallel make check

[VandeVondele Joost]

I've noticed that 

make -j -k check-fortran

results in a serialized checking, while 

make -j32 -k check-fortran

goes parallel. Somehow the explicit 'N' in -jN seems to be needed for the check target, while the other targets seem to do just fine. Is that a feature, or should I file a PR for that... ?

Somewhat related.... is there a rule of thumb on how is the granularity of parallel check decided ? E.g. check-fortran seems to be limited to about ~5 parallel targets, which is few for a typical server (but of course a welcome speedup already).

Thanks,

Joost