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

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@97672 13f79535-47bb-0310-9956-ffa450edef68