Ensure that AUTOBUILD is always in native path syntax on every platform.
Remove local build.sh logic to infer AUTOBUILD: either the generic build.sh
has set it properly, or scream for help.
Since cygwin bash scripts need a cygwin-syntax path to autobuild, convert to a
local 'autobuild' variable using shell_path.
Remove kludges in other languages involving cygdrive twiddling.

Hopefully this will make certain errors clearer, notably failure to launch
autobuild due to "cannot find the file specified".