cmake: Improve building with the system libpng.

This also now requires at least libpng16 instead of libpng17.

v2: Require libpng17 on arm platforms.
v3: Fix the android and windows travis builds.

Core/Debugger/WebSocket/GPUBufferSubscriber.cpp

@@ -17,7 +17,7 @@
 
 #include <algorithm>
 #ifndef USING_QT_UI
-#include <libpng17/png.h>
+#include <png.h>
 #include <zlib.h>
 #endif
 #include "data/base64.h"