ftp/ocaml-ocurl: unbreak the build under some circumstances

MAKE_JOBS_UNSAFE knob had been erroneously removed in d19bfbb151d2
(r477326) which had made the build fragile due to a race condition
when generating the `libcurl-helper.a' target. Clean the Makefile
up a bit while here and assume maintainership of the port.

Reported by: pkg-fallout