Remove just-installed su, if unable to make it set-UID root.

Remove just-installed su, if unable to make it set-UID root.

Normally, su isn't even installed.  However, if you configure with
--enable-install-program=su, and then install with insufficient
privileges, we now make sure to remove the just-installed binary.
* src/Makefile.am (install-exec-hook): Rename from install-exec-local,
so that this rule is guaranteed to be run *after* installation.
If unable to chown and chmod the installed "su" program, remove it.
Reported by Greg Schaefer.