Check for ICU Library

[Stefan Bidi]

I just wanted to make a note about the use of ICU in gnustep-base.  There has been some work done recently that uses unicode/unorm2.h, this header and it's API was introduced on ICU version 4.4 and did not exist on the version of ICU that configure checks for.  The check there needs to be updated for version 4.4 and above.  The version check update is easy to do, but I won't be home for another few hours.