Don't use obsolete AM_CONFIG_HEADER

Change-Id: Ie2744e3dee12361ae1a8c427c114a923a2c91060
Reviewed-on: http://review.couchbase.org/23800
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>