Made stripping of a trailing slash in directory names in top-level

configure script more robust and this way support also a plain `/' as
the argument without resulting in an empty name.

PR: 5291


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@84165 13f79535-47bb-0310-9956-ffa450edef68