[tlbuild] Tweak for harfbuzz

[Karl Berry]

Hi Ken and all,

    --- Build/source/libs/harfbuzz/configure.ac     (revision 46123)
    +++ Build/source/libs/harfbuzz/configure.ac     (working copy)
    ...
    -       CXXFLAGS="$CXXFLAGS -std=c++11"
    +       CXXFLAGS="$CXXFLAGS -std=c++11 -D_GNU_SOURCE"

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. :(.

Anyway, thanks. --karl