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/rotate.cc

@@ -24,7 +24,7 @@
 #include "lensgeomlistener.h"
 #include "rtimage.h"
 
-#include "../rtengine/procparams.h"
+#include "rtengine/procparams.h"
 
 using namespace rtengine;
 using namespace rtengine::procparams;