get "make extraclean" and probably "make distclean" to work on AIX

again...  $(RM) isn't defined everywhere (GNU make built-in?), so
invoke rm the way we invoke it elsewhere in the makefile


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94996 13f79535-47bb-0310-9956-ffa450edef68