This is so that it is easier for one building GTK+ with Visual Studio to
build the introspection files as well in one shot. Note that this is
not built by default, so one needs to select the gtk3-introspect
explicitly to build (and clean up) the introspection files.
For this to work, one needs to ensure the following:
-A complete build of GObject-Introspection in $(GlibEtcInstallRoot),
that is built with the same installation of the Python interpretor that
is used here (see PythonDir and PythonDirX64 in
-Introspection files for ATK, GDK-Pixbuf and Pango, also in their proper
locations under $(GlibEtcInstallRoot), which should be built with the
same G-I installation.