It's an external library, but it's a large C++ lib and it's built
using gbuild, so this helps too. Here with Clang it cuts build
time from ~2:40 to ~1:25 (and about halves the CPU user time).
Hopefully it also helps to reduce the build time for Windows gerrit builds.

Change-Id: Ieef0ea28e0fe1fd216592fca70410075d95ddc54
Reviewed-on: https://gerrit.libreoffice.org/72517
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>