Two issues with Cygwin hack about pdfjam

[lemniscati]

Thank you for pointing out!
I made those commits.

1. I'll make a pull-request to avoid using uname with -o option.
2. __cache__using_non_cygwin_latex_from_cygwin is stored as an exit status, and is expected to be used as

   return "${__cache__using_non_cygwin_latex_from_cygwin}"
   

   I agree that it is confusing, though...
   I have no idea, except replacing __cache__ by __exit_status__ or, so...

[rrthomas]

Thanks for the PR, @lemniscati.

[rrthomas]

Closing as fixed by #70.