Fix lookup of apu-config for VPATH builds

For included APU, apu-config is not in the srcdir but instead
in the builddir. That only matters for VPATH builds, ie. when
you build outside of the source tree.

Similar to r951769 (apr-config).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@951865 13f79535-47bb-0310-9956-ffa450edef68