Link curl and the test apps with -lrt explicitly when necessary

When curl calls a function from that library then it needs to
explicitly link to the library instead of piggybacking on
libcurl's own dependency.  Without this, GNU ld with the
--no-add-needed flag fails when linking (which Fedora now does
by default).

Reported by: Quanah Gibson-Mount
Bug: http://curl.haxx.se/mail/lib-2010-09/0085.html