Get rid of GTK dependencies

Build with aura by default and remove GTK related code.
This patch introduces a mechanism for setting gyp build
configuration variables using a GYP_CONFIG and adds a
desktop_linux.pri configuration for this target.

Change-Id: I6b044ba52ebb20c187572669af94a6fc280b0412
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>