Use GCC's warning key -Wzero-as-null-pointer-constant.

This flag will be useful for checking using new C++11 keyword nullptr. But
because this flag produce a lot "false alarm" from insides of Qt we can't
suppress better to wait time when Qt team will fully use Q_NULLPTR macros.

--HG--
branch : develop

common.pri

@@ -259,6 +259,11 @@ GCC_DEBUG_CXXFLAGS += \
     -Wno-unused \
     -ftrapv
 
+# Good support Q_NULLPTR come later
+greaterThan(QT_MAJOR_VERSION, 4):greaterThan(QT_MINOR_VERSION, 4) {
+GCC_DEBUG_CXXFLAGS += -Wzero-as-null-pointer-constant
+}
+
 # Since GCC 5
 g++5:GCC_DEBUG_CXXFLAGS += \
     -Wswitch-bool \