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/labcurve.cc

@@ -25,9 +25,9 @@
 #include "curveeditorgroup.h"
 #include "options.h"
 
-#include "../rtengine/color.h"
-#include "../rtengine/improcfun.h"
-#include "../rtengine/procparams.h"
+#include "rtengine/color.h"
+#include "rtengine/improcfun.h"
+#include "rtengine/procparams.h"
 #include "editcallbacks.h"
 
 using namespace rtengine;