Library: add new "define" tag

This tag will allow to add some preprocessor defs into library.
It would be useful to provide more information about libraries
implementation details. As example GLib's library include tag
was added that helps to detect more memory leaks.
Merge pull request #236 from scriptum/rpg-add-include-to-lib

Library: add new "define" tag
Merge branch 'master' into gui2