scons: Add SConscript file for building the libicu library.

This has been added as on FreeBSD platform it has to be build from source.
A version available in pkg repository uses libcxxrt ABI library, whereas
rest of the agent is build with libsubc++ that are not compatible with
each other and are causing the Agent to crash.
REBASE THIS: Netlink related incompatibilites
REBASE THIS: Coding differences between Freebsd and Linux.