When building modules with prereqs, search httpd include dirs

before prereq include dirs in case httpd is installed to the same
place as the prereq, in which case we might be using an older
httpd .h file.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1529452 13f79535-47bb-0310-9956-ffa450edef68