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

[Akira Kakuto]

Hi all,

> Harfbuzz 2.5.0 fails to compile with the centos7-provided compiler
> (which can compile everything else, including the parts that require
> C++11). I suppose it is some C++ feature that is not supported. Anyone
> up for researching if there's so`lme way we can tweak the source to make
> it work? I won't have time to look at it for a couple of
> days. --thanks, karl.

I have updated to harfbuzz-2.5.1 in r51283.
I expect that the situation is not changed.
It is highly desirable to obtain a workaround for systems like centos7.

Thanks,
Akira