Handle the libxml2 prereq of some modules, including its possible

dependency on iconv.  With that in place, build mod_proxy_html
and mod_xml2enc if libxml2 is found.

Don't reference zlib or OpenSSL-related variables set by the 
FindXXX macro unless they were actually found (avoids a cmake
warning).

Improve the readme.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1518185 13f79535-47bb-0310-9956-ffa450edef68