Set defaults for non-autoconf path variables. Also export a couple

Set defaults for non-autoconf path variables. Also export a couple
variables that for whatever reason weren't being placed into
config_vars.mk.

Warning: This commit alters the "default" layout (ie. when no --enable-layout
is specified). The most notable change will be htdocs in $prefix/share/htdocs,
which works with the autoconf defaults, but may not be what we want. This
will require more work.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93605 13f79535-47bb-0310-9956-ffa450edef68