[HarfBuzz] seems not to build without fallback shaper

[Martin Hosken]

Dear Behdad,

I recently had occasion to build harfbuzz without setting -DHAVE_FALLBACK and it didn't do too well.

I don't think it unreasonable to require the fallback shaper. But in that case, perhaps it's worth getting rid of the need for -DHAVE_FALLBACK.

Minor point, etc. But wanted to flag it in passing. And I could have been wrong.

Yours,
Martin