Fix --enable-debug regression.

Fix --enable-debug to actually enable debug mode.  This regression was
introduced by cbf3a6d703 (Move centralized
chunk management into arenas.).

configure.ac

@@ -634,6 +634,9 @@ fi
 ],
 [enable_debug="0"]
 )
+if test "x$enable_debug" = "x1" ; then
+  AC_DEFINE([JEMALLOC_DEBUG], [ ])
+fi
 AC_SUBST([enable_debug])
 
 dnl Only optimize if not debugging.