[tlbuild] Support for musl

[Karl Berry]

     > I'll see whether I can fix this somehow but I'm not an autotools
     > wizard.

I doubt Autotools are much of the issue. It's more patching the code to
work with the random assortment of headers on the given system, as has
already been discussed.

As of this year, C++11 is a fundamental requirement of ICU and Poppler.
(Not our choice.)

  John Calcote, Autotools, no starch press, July 2010,
       ISBN-13: 978-1-59327-206-7

2010 was a number of releases ago, although no doubt it's still valid,
since they do hard to keep compatibility. (Maybe not quite as hard as
us, but close. :)

FWIW, personally, I've found the autoconf and automake manuals
themselves to be pretty good. Much better than average. Real-life
examples as well as the reference, etc. Of course one has to be willing
to actually sit there and read and investigate :). --best, karl.