fdatasync: port to Solaris Cater to Solaris requiring extra libraries for fdatasync. * m4/fdatasync.m4 (gl_FUNC_FDATASYNC): Set LIB_FDATASYNC. * modules/fdatasync (Link): Document it. * modules/fdatasync-tests (test_fdatasync_LDADD): Link with it. Signed-off-by: Eric Blake <eblake@redhat.com>