[tlbuild] gcc vs. clang in icu

[Karl Berry]

    To Karl: users have been testing XeTeX for about a month 

Yes, I know that very well.

Like I said, it's a perfectly viable option to just continue to do what
has been done until and unless a problem arises.

    I would like to warn that there is a fair chance that ICU would try
    to be built with clang also on other platforms in case that clang
    exists.

Indeed, it will happen on any platform with clang(++) present.

k