Add compile time switch for HarfBuzz support

Add compile time switch for HarfBuzz support

Now that it's possible to switch shapers at runtime, it also becomes a
lot easier to disable them at compile time. Add ifdefs and build system
changes so that HarfBuzz can be safely disabled. It's autodetected now
and enabled if available.

This shuffles a bit of code around, but there are no functional changes.

Note that FriBidi remains mandatory for the time being, but this
shouldn't pose any problems, since it is a small and very portable
library without any special dependencies.