LU-2662 build: pthread build issue

Adding strlcpy to lustre/utils/liblustreapi.c caused
unexpected build issues.

The build would fail with undefined refrence to
'pthread_create` in user-prim.c.

Adding the pthread flags to the makefile allow the
build to past. I am not sure this is the "BEST" way.

Signed-off-by: Keith Mannthey <keith.mannthey@intel.com>
Change-Id: I62ac4d5c53a42411ef2e8ff37a455308cffed167
Reviewed-on: http://review.whamcloud.com/5180
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>