build: Add gengir.bat to build introspection for ATK.

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.