[CMake] Check whether C++ headers are self-sufficient

[Christoph Grüninger]

Hi Andreas,

> Why do you want to use one library per header? I think, it would be
> sufficient to have one cc-file per header and one library per folder.

Then I can test also for single headers.

> Why do you worry about large build directories?

Because for slow file systems like Windows it takes several minutes just
to create all the build folders and files.
When we have teaching sessions the computer pool uses some remote file
system (NFS?) which almost collapses if several participants configure
our software at the same time.

Bye
Christoph

-- 
When you die, that does not mean that you lose to cancer,
you beat cancer by how you live, why you live, and in the
manner in which you live.      -- Stuart Scott, 1965-2015