add option EXTRA_INCLUDE_DIRS for pointing to outside directories

for .h files (sometimes needed with statically linked, third-party
modules)

axe remnant of the old way of finding private APR .h files

install .h files that live outside of the top include directory


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1519175 13f79535-47bb-0310-9956-ffa450edef68