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/main.cc

@@ -42,8 +42,8 @@
 #include "rtimage.h"
 #include "version.h"
 #include "extprog.h"
-#include "../rtengine/dynamicprofile.h"
-#include "../rtengine/procparams.h"
+#include "rtengine/dynamicprofile.h"
+#include "rtengine/procparams.h"
 #include "pathutils.h"
 
 #ifndef _WIN32