AC_CHECK_FUNC doesn't automatically set HAVE_$function defines
(AC_CHECK_FUNCS does, confusing much?). Which means practically
speaking, the check is not useful. There _are_ #ifdef HAVE_FDATASYNC
in sqlite, but fdatasync is notably broken on older kernels, and while
lmdb has runtime workarounds for that, sqlite doesn't. It might be fine
to enable the sqlite code because those kernels are old, but this is
not a straightforward decision to make at the build system level, so
we'll handle that in a followup.

Differential Revision: https://phabricator.services.mozilla.com/D134855