FreeBSD: only assume iconv for FreeBSD >= 10

Since FreeBSD 10 is the oldest version of the OS supported by the
FreeBSD Project, we shouldn't need this.  But people are still using
older versions.  On closer examination, it's only been 6 weeks since 9.3
stopped being supported.  People ignoring the status are playing with
fire, getting no security updates, but let's not make that _our_
problem.

Guard the "use system iconv" #define for the libiconv package with an OS
version #ifdef.

(cherry picked from commit 852e25c532665a5523f07930030199666aef996c)
Signed-off-by: Phil Pennock <pdp@exim.org>