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

rtgui/pcvignette.cc

@@ -3,7 +3,7 @@
  */
 #include "pcvignette.h"
 
-#include "../rtengine/procparams.h"
+#include "rtengine/procparams.h"
 
 using namespace rtengine;
 using namespace rtengine::procparams;