Generate httpd.exp on the fly.

Generate httpd.exp on the fly.
This should allow DSOs to work on AIX, without the headache of maintaining
the httpd.exp file.
This is adapted from OS/2's generation of ApacheCoreOS2.def.
There exist a few bugs still:
    1) mod_dav and mod_proxy may not yet work, due to certain namespace issues.
    2) Some symbols may need to be added, a la core_header.def
Once these have been fixed, the old httpd.exp file will be deleted.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89288 13f79535-47bb-0310-9956-ffa450edef68