(mk_script): Set to ../mk-script.
(x-tests): Use `$(PERL) -w -- $(mk_script)', not ./mk-script.
Remove @MAINT@ cruft.
(Makefile.am): Likewise.
Remove @MAINT@ cruft. Now `missing' will explain the failure
when people don't have Perl yet modify a file whose rebuilding
would lead to the use of Perl.