Index: ruby-1.9.2-rc2/ext/dbm/extconf.rb
===================================================================
--- ruby-1.9.2-rc2.orig/ext/dbm/extconf.rb
+++ ruby-1.9.2-rc2/ext/dbm/extconf.rb
@@ -21,12 +21,12 @@ headers = {
 def headers.db_check(db)
   db_prefix = nil
   have_gdbm = false
-  hsearch = nil
+  db_defs = nil
 
   case db
   when /^db2?$/
     db_prefix = "__db_n"
-    hsearch = "-DDB_DBM_HSEARCH "
+    db_defs = %w[-DDB_DBM_HSEARCH -DHAVE_DBM]
   when "gdbm"
     have_gdbm = true
   when "gdbm_compat"
@@ -36,9 +36,9 @@ def headers.db_check(db)
   db_prefix ||= ""
 
   if (have_library(db, db_prefix+"dbm_open") || have_func(db_prefix+"dbm_open")) and
-      hdr = self.fetch(db, ["ndbm.h"]).find {|h| have_type("DBM", h, hsearch)}
+      hdr = self.fetch(db, ["ndbm.h"]).find {|h| have_type("DBM", h, db_defs.join(" "))}
     have_func(db_prefix+"dbm_clearerr") unless have_gdbm
-    $defs << hsearch if hsearch
+    $defs += db_defs if db_defs
     $defs << '-DDBM_HDR="<'+hdr+'>"'
     true
   else