Configurations/unix-Makefile.tmpl: fix HP-UX build.

HP-UX make doesn't recognize $< in explict target rules, only in
inference ones such as .c.o.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4697)