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/filecatalog.cc

@@ -26,8 +26,8 @@
 
 #include <glib/gstdio.h>
 
-#include "../rtengine/rt_math.h"
-#include "../rtengine/procparams.h"
+#include "rtengine/rt_math.h"
+#include "rtengine/procparams.h"
 
 #include "guiutils.h"
 #include "options.h"