Fix recent exports.c breakage by referencing exports.c as being in

top_builddir rather than top_srcdir.

For VPATH builds, we must treat exports.c as being in the build tree rather
than in the source tree.  Otherwise, it won't be regenerated properly.

(FreeBSD's make does not require the explicit path, so I'm not sure why this
change was made at all.)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97144 13f79535-47bb-0310-9956-ffa450edef68