Make HarfBuzz compile on any platform using the --with-harfbuzz
option. Support is experimental only.

Change-Id: I84fb80f3f8abed8ac877a294cf7ef39cf4cb2e9e
Reviewed-on: https://gerrit.libreoffice.org/25369Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>
Tested-by: Khaled Hosny <khaledhosny@eglug.org>