change location of headers for iOS static lib target

loosen the requirement that an application/super-project must reference the subproject headers explicitly through the source tree; also avoids copying the header into an application archive; see jamie James Montgomerie’s explanation here: http://www.blog.montgomerie.net/easy-xcode-static-library-subprojects-and-submodules
Merge remote-tracking branch 'pizthewiz/target-header-location'