g-ir-scanner --no-libtool needed some fixes similar to [1] for Cygwin, as
well. Now that is done, it's possible to make these tests run and pass on
Cygwin.
[1] https://bugzilla.gnome.org/show_bug.cgi?id=781525
These paths are now generated similar to
NinjaBackend.generate_single_compile where IncludeDirs create includes
of both the build directory path and the source directory path.
This also fixes a bug with include_directories, where the path string
supplied to the IncludeDirs initializer was used for the search path,
instead of the actual location to which it referred. Often, this was a
'.', and not a really useful path.