Makefile: avoid use of GNU-specific form of $<

$< is only allowed in implicit rules in some non-GNU makes (e.g. BSD,
AIX) so avoid use elsewhere by referencing the dependent curl.1 file
directly instead. This is somewhat tricky because the file is supplied
in the packaged tar ball (but not in git) but must still be able to be
rebuilt when its dependencies change. The right thing must happen in
both tar ball and git source trees, as well as in both in-tree and
out-of-tree builds.