Made `tr' usage in top-level configure script more portable

by always using square brackets consistently.

Submitted by: Masashi Kizaki <kizaki@cpo.dnp.co.jp>
Reviewed by: Ralf S. Engelschall
PR: 5230


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@84164 13f79535-47bb-0310-9956-ffa450edef68