Clean up some of the includes:

- explicitly include apr_lib.h since ap_config.h doesn't
- use apr_want.h where possible
- use APR_HAVE_ where possible
- remove some unneeded includes


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88060 13f79535-47bb-0310-9956-ffa450edef68