Use AC_CHECK_HEADER for memcached/engine.h
Use AC_CHECK_HEADER for memcached/engine.h

AC_CHECK_HEADER_ONCE won't work unless we find an installed
version of memcached/engine.h under $(PREFIX)/include

Change-Id: I408a153c43e038e17c06e9013519445ad24f53ff
Reviewed-on: http://review.couchbase.org/7069
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>
Reviewed-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

daverigby/kv_engine,teligent-ru/ep-engine,abhinavdangeti/ep-engine,daverigby/kv_engine,hisundar/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,couchbaselabs/ep-engine,couchbase/ep-engine,teligent-ru/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,daverigby/kv_engine,hisundar/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,sriganes/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,daverigby/ep-engine,sriganes/ep-engine,abhinavdangeti/ep-engine,couchbaselabs/ep-engine,abhinavdangeti/ep-engine,owendCB/ep-engine,couchbase/ep-engine,couchbase/ep-engine,couchbaselabs/ep-engine,membase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,couchbaselabs/ep-engine,hisundar/ep-engine,owendCB/ep-engine,abhinavdangeti/ep-engine,membase/ep-engine,daverigby/ep-engine,jimwwalker/ep-engine,sriganes/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine