Add configure support for HarfBuzz.

* builds/unix/pkg.m4: New file.
* builds/unix/configure.raw: Search for libharfbuzz using
`pkg-config'.
Add `--without-harfbuzz' option.
* builds/unix/freetype-config.in, builds/unix/freetype2.in,
builds/unix/unix-def.in (freetype-config, freetype2.pc): Handle
HarfBuzz.

* docs/INSTALL.UNIX: Document interdependency of Freetype with
HarfBuzz.
7 files changed