Mark library include dirs as SYSTEM.

Compilers will suppress warnings from files in those directories.

src/core/Endian.cpp

@@ -17,14 +17,7 @@
 
 #include "Endian.h"
 
-#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 3))
-#pragma GCC push_options
-#pragma GCC diagnostic ignored "-Wold-style-cast"
-#endif
 #include <QtCore/QtEndian>
-#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 3))
-#pragma GCC pop_options
-#endif
 #include <QtCore/QIODevice>
 
 namespace Endian {