* not depending on "-dev" packages for the built Debian package
* some refactoring in if-clauses and with "quotes"
* libvterm dependency: if something is missing, made clear what
Some other changes to the CMake-files done as well:
* documentation looks more like Doxygen-style
* prevent mix-up with C++ GSL library ("Guidelines support library" not
"GNU scientific library")
There is no version detection. If there is any vterm library, it is
used. If it is the wrong version, there will be an build error.
`libvterm.so.0.0.2` is preferred, which hopefully is compatible to
revision 740 or Ubuntu `libvterm0` version 0.1.2-2.
If no library is found, the correct one is downloaded and build.