[tlbuild] Tweak for harfbuzz

[Akira Kakuto]

Hi Karl,

> Sure, inserted. Though it puzzles me that the original source apparently
> does nothing to force either -std=c++11 or -D_GNU_SOURCE. I guess it's
> because they see C++11 as "optional". I can't remember why we needed to
> require it, can't research further now. :(.

I find in the ChangeLog:

Date:   Sun Oct 15 12:12:10 2017 +0200 (1.6.1 ?)

    Use C++11

Date:   Fri Oct 27 10:29:25 2017 -0600 (1.7.0 ?)

    Make c++11 optional

Best,
Akira