build: work around lack of MacOS fdatasync

build: work around lack of MacOS fdatasync

Mingw lacks fsync, but gnulib provides that.  Meanwhile, gnulib does
not (yet) provide fdatasync, so this is a quick hack to fake that
function on MacOS X; we can revert this configure change once gnulib
gives us a real module.

We have been implicitly relying on gnulib's largefile module being
pulled in by other modules, but it's better to make that explicit.

* bootstrap.conf (gnulib_modules): Add fsync.  Make largefile use
explicit.
* configure.ac (AC_CHECK_FUNCS_ONCE): Check for fdatasync, and
fake it with fsync when not present.