Minor fix which helps Xcode generated project build.
Minor fix which helps Xcode generated project build.

Issue is that the add_custom_command references gencode/genconfig via
a path, yet on Xcode build all output goes into
build/ep-engine/Debug|Release/ thus gencode/genconfig fail dependency
checks.

Using just the add_excutable name of gencode/genconfig allows cmake
to use the correct path.

Change-Id: Ie55124487bffe80bc2847b67977738e67d8da855
Reviewed-on: http://review.couchbase.org/40825
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

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