We don't want to clutter /usr/bin/ with internal helpers, do we?
On start-up, the libexec directory is added to the PATH environment
variale. This allows reusing exec.LookPath logic, notably, to check
whether the script is executable.
It is separate from the PREFIX variable which specifies the path use
*within* the system tree whereas DESTDIR specifies the path to the
system tree itself. For get.sh-based installation, DESTDIR=""
PREFIX=/usr/local. For package.sh DESTDIR is a temporary directory and
PREFIx=/usr.
With that change it is now possible to set the default libexec directory
to a correct value depending on the PREFIX variable.
