Change gating condition for likely/unlikely definitions

Currently, likely and unlikely defines are gated by __GLIBC__.
This define isn't a problem until building within nfs-ganesha as
nfs-ganesha uses (__GNUC__ >= 3).
Therefore, change tirpc to use the same gating define as nfs-ganesha.
Signed-off-by: Viji Ganapathi <vganapathi@panasas.com>