update to icu has again broken all the macports-clang compilers

[Ken Cunningham]

the macports-clang compilers actually have no need for macports' libxml2 I believe, the libxml2 provided by the system is sufficient, and libxml2 itself is optionally used by one tiny component that is not relevant to a bootstrap compiler...

I have disabled libxml2 in various llvm-related ports already, so perhaps I should just look at removing our libxml2 from all the llvm* ports and maybe solve this forever.

Ken