make a proper header-only module for QtZlib

that way other modules can use the headers without hacks.

this required making the base directory for paths in headers.pri
configurable in syncqt.

Change-Id: Id35cfe05bcf4c576d3f2d0d8d09590a5e23d21d3
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>