[tex-live] ICU code added, configure/makefile updated

[Jonathan Kew]

FYI -

I have added the ICU library code (used for xetex) to the TeX Live  
source tree. This is a slightly modified version of the ICU 3.4  
release, so I've added it as libs/icu-xetex to indicate that it is  
not exactly as released. (See also libs/howto/icu-xetex.)

I have also updated the top-level configure and make scripts so that  
the ICU libraries will be built as part of the standard ./Build  
process. This seems to work fine for me on Mac OS X and Linux....  
YMMV. If these changes lead to build problems for anyone, please let  
me know and we'll try to resolve whatever issues there may be.

(./Build won't try to compile xetex itself, as not all the sources  
are present yet. But I'm gradually trying to get all the pieces in  
place....)

JK