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/demosaic_algos.cc

@@ -23,7 +23,7 @@
 #include "rawimage.h"
 #include "rt_math.h"
 #include "color.h"
-#include "../rtgui/multilangmgr.h"
+#include "rtgui/multilangmgr.h"
 #include "opthelper.h"
 #include "median.h"
 //#define BENCHMARK