If apr and apr-util are not in-tree, we need to be able to find the

  include directory for each in order to generate the server/exports.c
  and server/export_vars.h files.

  configure.in:
   - Provide APR_INCLUDEDIR, APU_INCLUDEDIR.

  server/Makefile.in:
   - Use $APR_INCLUDEDIR $APU_INCLUDEDIR vars.
   - Add export_files target to generate a list of headers with symbols
   to export, use that list to generate exports.c and export_vars.h.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97673 13f79535-47bb-0310-9956-ffa450edef68