In one target we weren't always creating the $libexecdir

In one target we weren't always creating the $libexecdir
($prefix/modules) directory, and in another we were assuming that the
directory existed. Now we always create it in the first target, and we
check that it exists in the second.

This fixes a bug where httpd.exp would be installed as a _file_ called
$prefix/modules.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91492 13f79535-47bb-0310-9956-ffa450edef68