Fix apxs to allow it to work when the build directory is somewhere

besides server-root/build.

PR:           8453

Submitted by: a number of people hit this problem and offered hints or
              partial fixes; Thom May and Pier Fumagalli had a patch to
              apxs, a subset of which mostly fixed this problem; there
              were hints in PRs 8453 and 9316


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95522 13f79535-47bb-0310-9956-ffa450edef68