- add some missing distclean targets

- use DISTCLEAN_TARGETS and CLEAN_TARGETS everywhere (to be consistent
with other macros and with APR)
- warn that aclocal.m4 is a generated file


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87597 13f79535-47bb-0310-9956-ffa450edef68