[Emacs-ada-mode] update including gnatpath source

[Stephen Leake]

I've uploaded a new version of Emacs Ada mode. 

This includes the sources for gnatpath.

I also allow "$var" syntax for environment variables more consistently
in Ada mode project variables.

The Windows binary for gnatpath is also on the website. I don't have a
lot of room, but if there is sufficient demand, I could host some
other binaries as well.

-- 
-- Stephe