work around an AIX "issue" in the construction of config_vars.mk by

modifying the sed expression just a bit;

note that before and after this change manualdir isn't being handled
properly; using the default Apache layout we get

  manualdir = $datadir/manual

instead of

  manualdir = $(datadir)/manual

Submitted by:	Victor Orlikowski, after I narrowed down the issue


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90930 13f79535-47bb-0310-9956-ffa450edef68