Fix program build rule in src/bin/scripts/Makefile.

Fix program build rule in src/bin/scripts/Makefile.

Commit 69ae9dcb4 added a globally-visible "%: %.o" rule, but we failed
to notice that src/bin/scripts/Makefile already had such a rule.
Apparently, the later occurrence of the same rule wins in nearly all
versions of gmake ... but not in the one used by buildfarm member jacana.
jacana is evidently using the global rule, which says to link "$<",
ie just the first dependency.  But the scripts makefile needs to
link "$^", ie all the dependencies listed for the target.

There is, fortunately, no good reason not to use "$^" in the global
version of the rule, so we can just do that and get rid of the local
version.