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

rtengine/iccstore.cc

@@ -38,8 +38,8 @@
 #include "iccmatrices.h"
 #include "utils.h"
 
-#include "../rtgui/options.h"
-#include "../rtgui/threadutils.h"
+#include "rtgui/options.h"
+#include "rtgui/threadutils.h"
 #include "lcms2_plugin.h"
 
 #include "color.h"