Visual Studio builds: Adapt to the Python-fied glib-genmarshal

Visual Studio builds: Adapt to the Python-fied glib-genmarshal

glib-genmarshal is used for the Visual Studio builds, and has been
recently converted to a Python script instead of a compiled C program.

Since Visual Studio calls cmd.exe to run Custom Build steps, we need
to call Python explicitly to run glib-genmarshal as shebang lines
are not supported in cmd.exe, which means that Python will become a
hard build-time requirement instead of being optional.  This also means
that the atk.pc pkg-config file will always be generated.

However, since ATK does not yet require a really recent GLib, maintain
compatibility with older GLib by calling glib-genmarshal.exe when the
glib-genmarshal Python script is not found.