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/controlspotpanel.cc

@@ -18,11 +18,11 @@
  *  2018 Pierre Cabrera <pierre.cab@gmail.com>
  */
 
-#include "../rtengine/rt_math.h"
+#include "rtengine/rt_math.h"
 #include "controlspotpanel.h"
 #include "editwidgets.h"
 #include "options.h"
-#include "../rtengine/procparams.h"
+#include "rtengine/procparams.h"
 #include "rtimage.h"
 #include "eventmapper.h"