[tlbuild] icu-65.1 in TL and -lpthread

[Karl Berry]

I've updated the ICU in TeX Live to ICU 65.1 (r53103). I surmise it is
likely to break the build on some systems. Please give it a try if you can.

In my testing, everything compiled and linked ok, but upmendex crashed
at runtime in an internal C++ function (std::call_once). This apparently
happened because we did not link against -lpthread (except on OpenBSD).
(Details in upmendex/ChangeLog.)

Even now, the other ICU-using programs (bibtexu and xetex) do not seem
to need -lpthread. Or we are not testing the right things that would
cause the problem. I don't know, but I don't want to gratuitiously add a
dependency on -lpthread.

So now upmendex links against -lpthread if it is available, but the
other ICU programs still do not. If this turns out to be needed, it
will be easy enough to adjust.

Also, from looking at the autoconf-archive AX_PTHREAD macro, I know that
the best threading library might not actually be named -lpthread and
might need other arguments. We can use it if we have to. I just hoped
the simple test I put in for a -lpthread that provides pthread_create()
will suffice in our case.
(https://www.gnu.org/software/autoconf-archive/ax_pthread.html)

Fingers crossed,
Karl