Fix a bug that would cause SERVER_CONFIG_FILE to be an absolute path,

which would prevent the relocation of an httpd binary without having to
specify the -f flag.

Now it will be relative to the $prefix if $sysconfdir shares a common prefix,
or it will remain relative to $sysconfdir.

Submitted by:	Thom May <thom@planetarytramp.net>, Aaron Bannert


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93798 13f79535-47bb-0310-9956-ffa450edef68