[tlbuild] icu-65.1 in TL and -lpthread

[Karl Berry]

    It fails to build on Solaris and on an older Mac. On Mac there is a
    Python-related issue, on Solaris it's C++.

Sorry but not surprised to hear it.

    https://build.contextgarden.net/#/waterfall?tags=c.texlive
    (Writing from my phone, cannot investigate right now.)

I hope you or someone will be able to investigate soon. My energy for
dealing with ICU's endless portability problems is lacking at the moment :(. -k