Quote variables in -z tests correctly in wx-config.

The changes of r69944 broke wx-config for the traditional Unix systems (at
least Solaris and AIX, probably others too) which don't support using -z
without any value (unlike Linux/bash).

Fix this by quoting the possible empty variables in the tests.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69969 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

wx-config.in

@@ -1057,11 +1057,11 @@ order_libs()
 
         # Add the libraries that we postponed adding above.
         # Order of the checks here is important.
-        [ -z $use_html ] || libs="$libs html"
-        [ -z $use_adv ] || libs="$libs adv"
-        [ -z $use_core ] || libs="$libs core"
-        [ -z $use_xml ] || libs="$libs xml"
-        [ -z $use_base ] || libs="$libs base"
+        [ -z "$use_html" ] || libs="$libs html"
+        [ -z "$use_adv" ] || libs="$libs adv"
+        [ -z "$use_core" ] || libs="$libs core"
+        [ -z "$use_xml" ] || libs="$libs xml"
+        [ -z "$use_base" ] || libs="$libs base"
     else
         # No need to order them.
         libs="$@"