math/vowpal_wabbit: fix build on 15-CURRENT

Link with -lutil to get ftime() which was moved there recently.
This is harmless on older FreeBSD. Define LICENSE while we are at it.

Approved by: portmgr (build fix blanket, infrastructure blanket)
MFH: 2024Q4

(cherry picked from commit ef3e5f26ff33dee71a04df95fed4dc8732638a08)