start using a config_header

populaing a "kv_config.h" makes it easier to inspect what configure
thinks it found vs. looking through compiler command lines.

configure.ac

@@ -7,6 +7,7 @@ AM_INIT_AUTOMAKE([1.14.1 -Wall -Werror foreign subdir-objects silent-rules])
 AM_SILENT_RULES([yes])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_SRCDIR([src/sds-keyval.h])
+AC_CONFIG_HEADERS([src/kv-config.h])
 LT_PREREQ([2.2])
 # LT_INIT needs to know about AR
 AM_PROG_AR

src/datastore.cc

 // Copyright (c) 2017, Los Alamos National Security, LLC.
 // All rights reserved.
 #include "datastore.h"
+#include "kv-config.h"
 #include <chrono>
 #include <iostream>
 

src/datastore.h

 // Copyright (c) 2017, Los Alamos National Security, LLC.
 // All rights reserved.
-#if BWTREE
+#include "kv-config.h"
+#if USE_BWTREE
 #include "bwtree.h"
 #endif
 #include "sds-keyval.h"
 
+
 #include <boost/functional/hash.hpp>
 #include <vector>
 #if LEVELDB

src/kv-server.cc

 #include "sds-keyval.h"
+#include "kv-config.h"
 #include "datastore.h"
 
 #include <mercury.h>

src/kvgroup-client.cc

 #include "sds-keyval-group.h"
+#include "kv-config.h"
 
 #include <stdlib.h>
 #include <time.h>