[configure] don't use relative include paths in addon headers

Preparing for binary addons, we now have addon dev packages on linux, e.g. https://github.com/xbmc/xbmc-packaging/commit/00b1b93ee7712ddf33d1acfccf42c55e258bd99a

For this to work the header cannot use relative include paths 

tested on linux only, VS project files are untested and Xcode is missing completely(plz help)

ping @cptspiff @opdenkamp