Only recompile buildmark.c when we have to relink httpd.

This allows us to be able to do a make on an up-to-date tree without forcing
a recompile and relink.  Yay!


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@153266 13f79535-47bb-0310-9956-ffa450edef68