[meson] Fix handling of HarfBuzz library. * meson.build (harfbuzz_dep): Do not fall back to HarfBuzz by default. Otherwise it causes a dependency cycle: cairo => fontconfig => freetype2 => harfbuzz => cairo Meson will still fall back to HarfBuzz subprojects if the `harfbuzz` option is set to `enabled` instead of `auto` and a `subprojects/harfbuzz.wrap` file is present. In that case it is the responsibility of the main project to set the proper options on each subproject to break the dependency cycle. Fixes: #1028.