Don't probe extra libraries for fdatasync.

Don't probe extra libraries for fdatasync.

Commit d2e15083 got rid of the main configure probe and HAVE_FDATASYNC
macro, but we still searched -lrt and -lposix4 for old Solaris systems.
It's in the C library on modern Solaris, as on other supported systems.

Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://postgr.es/m/CA%2BhUKGLLVx4drdvXats9PxH3eeB%2BE2NkJReJ%2BRGmaOpU%3D9rgEA%40mail.gmail.com