Update paths to ilclient and vgfont in Makefile.include

If a user wants to make an application based on the hello_pi example code, he/she may want to use Makefile.include as per the sample code. Currently this works, provided that the user puts the source of the new application in the /opt/vc/src/hello_pi/ tree, or the user does not use ilclient and vgfont. If the user includes either of those libraries when the project is not in said filesystem tree, the compiler will not find the library because Makefile.include specifies a relative path to the libraries.

This pull request makes paths to ilclient and vgfont absolute so Makefile.include can be included from other locations in the filesystem.