ICUDIR not necessary on some platforms

[isc-woodfin]

On some platforms (e.g. a typical Linux distribution), the default package manager will install ICU into the default include/library paths, which makes checking for ICUDIR redundant and indeed impossible to satisfy, as there's no "root" of ICU.

The root Makefile needs some basic logic to construct the ICU lib/include flags (which may both be null in the typical Linux case)

[isc-woodfin]

Alternatively we could rewrite the iKnow build to use autoconf but :-P