[tlbuild] more c++ agony, now with harfbuzz

[Karl Berry]

    > Harfbuzz 2.5.0 fails to compile with the centos7-provided compiler

For the record, being discussed at
https://github.com/harfbuzz/harfbuzz/issues/1724.  Other people before
us reported it. Don't know if a fix will come.  The problematic C++
feature in question remains obscure to me :(, so I reiterate my plea for
anyone else to take a look. --thanks, karl.