invoking gunzip.exe

[Eric Blake]

Texinfo makes a bad assumption that if gzip.exe exists, then gunzip.exe also 
exists.  On recent cygwin, this is not true (with gzip 1.3.9, gunzip was merely 
a symlink to the real gzip.exe, and the cygwin port of coreutils no longer 
puts .exe suffixes on symlinks; and with gzip 1.3.12, gunzip is a shell script 
and must not have the .exe suffix).

See also:
http://cygwin.com/ml/cygwin/2007-04/msg00680.html
http://cygwin.com/ml/cygwin/2007-04/msg00693.html

I'm not sure what patch Chris Faylor put in his latest cygwin drop of texinfo 
to fix this improper Windows-ism, but it would be nice to have upstream:
http://cygwin.com/ml/cygwin-announce/2007-04/msg00035.html

-- 
Eric Blake