linux-user: fix utimensat

The glibc function for utimensat glibc returns -EINVAL when the path is null
which is a different behaviour with the syscall.

path can be null because internally the glibc is using utimensat with
path null when implmenting futimens. If path is null, call futimes
instead.

don't try to copy timespec from user if is NULL.

Add configure check for older systems

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
