Use rsync to install manual and htdocs to speed up installation times.

* Makefile.in: Update install-man and install-htdocs to use rsync if available,
  otherwise fail back to current cp -rp and rm -rf .svn/ strategy.
* configure.in: Look for rsync.
* acinclude.m4: Include RSYNC in exported config_vars.mk

(Incorporates suggestions on rsync flags from colm.)

Reviewed by: colm, trawick (concept), niq (concept)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@292718 13f79535-47bb-0310-9956-ffa450edef68