add an <IfFile> config section like <IfDefine>

add an <IfFile> config section like <IfDefine>

It allows a non httpd config file to be used as a marker directly in
httpd.conf without hiding logic in a script in front of apachectl
to do test -f and pass extra -D's.

This is something we've had in IBM's httpd distro for a little bit and
hadn't remembered to share. I've seen some questions/config files come
up in a few places lately that would benefit from this as an option.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769718 13f79535-47bb-0310-9956-ffa450edef68