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/threadutils.h

@@ -27,7 +27,7 @@
 #include <mutex>
 #include <thread>
 #include <condition_variable>
-#include "../rtengine/noncopyable.h"
+#include "rtengine/noncopyable.h"
 
 #if STRICT_MUTEX && NDEBUG
 using MyMutexBase = std::mutex;