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
gtk3-version-paths.[vsprops|props]).
-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.
Update the autotools scripts so that we can support Visual Studio 2017
by copying the 2010 projects and updating items as needed to obtain
the 2017 projects.
Note that since the toolset version string changed for Visual Studio
2017, so allow the use of a custom toolset version string, otherwise
just generate the toolset version string as we did before.
Also, note that Visual Studio 2017 aims to be compatible with 2015
on the CRT level, so there should not be any problems using 2017-compiled
binaries with 2015-compiled ones.
