Add a new way to document changes

Add a new way to document changes

* Makefile.in: Define the update-changes target to merge the change files
  from the changes directory to the top of the CHANGES file and remove
  them afterwards. If a Subversion binary was found during configure try
  to svn rm them otherwise just delete them.

* configure.in: Search for the Subversion binary.

* acinclude.m4: Set the corresponding Makefile variable for the Subversion
  binary.

* README.CHANGES: New file to document the new approach.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1879822 13f79535-47bb-0310-9956-ffa450edef68