Get rid of relative include paths

* Use target_include_directories to specify include paths
* Specify project root (parent of rtgui and rtengine) as include path
* Replace relative includes with normal includes

rtengine/imageio.cc

@@ -52,8 +52,8 @@
 #include "settings.h"
 #include "utils.h"
 
-#include "../rtgui/options.h"
-#include "../rtgui/version.h"
+#include "rtgui/options.h"
+#include "rtgui/version.h"
 
 
 using namespace std;