Add support to set variables with the 'Define' directive. Change

ap_resolve_env() to look for variables defined in this way. This allows to use
the variables in the config using the ${VAR} syntax known from envvar
interpolation


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1061465 13f79535-47bb-0310-9956-ffa450edef68