Try to determine the location of erl_driver.h at configure time
Try to determine the location of erl_driver.h at configure time

Use the existence of erl in path to try to figure out where the
file should be located unless the user specified it.

Patch by Trond Norbye.

Closes COUCHDB-162

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb