By default, on all platforms except MinGW, libXpm will detect if a
filename ends in .Z or .gz, and will when reading such a file fork off
an uncompress or gunzip command to read from via a pipe, and when
writing such a file will fork off a compress or gzip command to write
to via a pipe.

In libXpm 3.5.14 or older these are run via execlp(), relying on $PATH
to find the commands.  If libXpm is called from a program running with
raised privileges, such as via setuid, then a malicious user could set
$PATH to include programs of their choosing to be run with those
privileges.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>