0001: There is no point in searching -lrt and -lposix4 for fdatasync()
if it's supposed to help Solaris only. They moved all the realtime
stuff into the main C library at least as far back as Solaris
10/OpenSolaris.
0002: There is no point in probing -lposix4 for anything. That was
superseded by -lrt even longer ago.
We could go further: I suspect the only remaining -lrt search we
still need in practice is for shm_open() on glibc < 2.34, but here I
just wanted to clean out some old Sun stuff that was called out by
name.