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