build: Add gengir.bat to build introspection for ATK.
This adds a Windows .bat file to build the introspection files for ATK,
which requires:
-An MSVC-built version of ATK
-A valid build of gobject-introspection, preferably done with MSVC.
-A MinGW GCC installation to run the pre-processor during the dumping
stage*
-A Python installation
-Properly configured set of pkg-config (.pc) files for GLib and its deps.
This does not require a MSYS/Cygwin/BASH shell installed on the Windows
system
*Note that the generated and built dumper program is still compiled and
linked with the Microsoft compiler.