Add LDFLAGS to PROG_VARS so it can be overridden on a per-PROG basis

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272917 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f