Merge r1872042, r1872062 from trunk:

Merge r1872042, r1872062 from trunk:

Add support for cross compiling to apxs. If apxs is being executed from somewhere
other than its target location, add that prefix to includes and library directories.
Without this, apxs would fail to find config_vars.mk and exit.

Allow instdso.sh to be found while cross compiling.

Submitted by: minfrin
Reviewed by: minfrin, jim, ylavic

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1874288 13f79535-47bb-0310-9956-ffa450edef68