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/dual_demosaic_RT.cc

@@ -31,7 +31,7 @@
 #include "rt_math.h"
 #include "rtengine.h"
 
-#include "../rtgui/options.h"
+#include "rtgui/options.h"
 
 using namespace std;