Introduce png_dependency.pri

Just like any other x_dependency.pri, it shall be included instead of
linking with png lib directly.

Change-Id: I698c8a8137f937d2e2c5f5dc9b9188b780c9e49a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

src/3rdparty/png_dependency.pri

+contains(QT_CONFIG, system-png) {
+    unix|mingw: LIBS_PRIVATE += -lpng
+    else: LIBS += libpng.lib
+} else: contains(QT_CONFIG, png) {
+    include($$PWD/libpng.pri)
+}

src/gui/image/qpnghandler.pri

 HEADERS += $$PWD/qpnghandler_p.h
 SOURCES += $$PWD/qpnghandler.cpp
-contains(QT_CONFIG, system-png) {
-    if(unix|mingw): LIBS_PRIVATE  += -lpng
-    else:win32:          LIBS += libpng.lib
 
-} else {
-    include($$PWD/../../3rdparty/libpng.pri)
-}
+include($$PWD/../../3rdparty/png_dependency.pri)