Unbreak the build when the ICU option is unset on FreeBSD 9.3.

USES must be set before including bsd.port.pre.mk, which is included
by boost-all/compiled.mk. The easiest fix for the conditional
USES+=iconv being too late seems to be to use an option helper.
For consistency use an option helper for the LIB_DEPENDS.

PR: 203285