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

Configurations/unix-Makefile.tmpl

@@ -856,7 +856,7 @@ EOF
           }
           return <<"EOF";
 $args{src}: $args{generator}->[0] $deps
-	\$(CC) $incs \$(CFLAGS) -E \$< | \\
+	\$(CC) $incs \$(CFLAGS) -E $args{generator}->[0] | \\
 	\$(PERL) -ne '/^#(line)?\\s*[0-9]+/ or print' > \$@
 EOF
       }