The config.h header for ngspice is not packaged with all platforms so
use pkg-config to fetch the version information when available. When
pkg-config is not available, check if the ngspice/config.h file is
installed on the system before including it to build the ngspice version
string. If neither pkg-config or ngspice/config.h is available set the
version string to "unknown".
Fixes https://gitlab.com/kicad/code/kicad/issues/4851
Recent msys version does not find libngspice-0.dll, only
libngspice-0.dll.a when using find_library.
When not found, a find_file is made, instead of find_library.
Some developer scripts ended up in the demonstration scripts directory
that was bundled with builds. This moves them to the build_tools
directory and updates the associated Documentation
Fixes: lp:1815891
* https://bugs.launchpad.net/kicad/+bug/1815891
Adds a link-time dependency for libngspice, so that other tools may
detect ngspice as a KiCad dependency.
The library is still loaded with dlopen() as it gives a way to reload it
in case of problems. The DLL name is recognized during CMake
configuration and used to load the library at runtime.