* src/smooth/ftgrays.c: Fix compilation if `FT_LONG64` is undefined. The code assumed that if `__SSE2__` is defined, then 64-bit integer types are available. This is not the case apparently for certain multilib compiler targets like 'x86_32.x86' used by Gentoo. This patch fixes the issue by disabling the special code path when 64-bit integer types are not available. Fixes #1071.