cpluspluscheck vs vpath

Hi,

right now cpluspluscheck doesn't work with vpath builds. That's pretty
annoying, because it does require cloning the git tree into a separate
directory + doing configure there just to run cpluspluscheck.

Attached is a small patch allowing cpluspluscheck to run from different
directories. I needs the src and build directories for that,
unsurprisingly.

As that makes it more complicated to invoke, I added a makefile target
(in the top level) for it.

Seems we could round the edges a good bit further than what's done in
the attached (argument checking, for example). But I think this would
already be an improvement?

Greetings,

Andres Freund