Added exp_foo and rel_foo variables to config_vars.mk for all Apache and

Added exp_foo and rel_foo variables to config_vars.mk for all Apache and
Autoconf path variables (like --sysconfdir, --sbindir, etc). exp_foo is
the "expanded" version, which means that all internal variable references
have been interpolated.  rel_foo is the same as $exp_foo, only relative
to $prefix if they share a common path.

This will be used in apxs.in and apachectl.in to allow Apache installs
to be relocated as long as the same relative directory structure remains.
Currently this is only possible if you use the Apache layout, but not
with other layouts or any overridden --foodir paths.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94108 13f79535-47bb-0310-9956-ffa450edef68