curl: Do not use 'dl' on HP-UX

Re-apply the logic change made by commit v2.8.0~1427 (fix warning on
HPUX, 2008-11-26).
curl: Simplify our LIBCURL test addition
Merge topic 'update-curl' into next

7b07b100 curl: Simplify our LIBCURL test addition
4fc9c22f curl: Do not use 'dl' on HP-UX