Define wxHAS_NATIVE_ANIMATIONCTRL

This allows to easily test for the native control existence and also
whether wxGenericAnimationCtrl is really different from wxAnimationCtrl.

It also allows to avoid explicit check for wxGTK in common code.

docs/doxygen/mainpages/const_cpp.h

@@ -185,6 +185,7 @@ Currently the following symbols exist:
     decide whether some function should be overloaded for both
     <code>long</code> and <code>long long</code> types.}
 @itemdef{wxHAS_MULTIPLE_FILEDLG_FILTERS, Defined if wxFileDialog supports multiple ('|'-separated) filters.}
+@itemdef{wxHAS_NATIVE_ANIMATIONCTRL, Defined if native wxAnimationCtrl class is being used (this symbol only exists in wxWidgets 3.1.4 and later).}
 @itemdef{wxHAS_NATIVE_DATAVIEWCTRL, Defined if native wxDataViewCtrl class is being used (this symbol only exists in wxWidgets 3.1.4 and later).}
 @itemdef{wxHAS_NATIVE_WINDOW, Defined if wxNativeWindow class is available.}
 @itemdef{wxHAS_NULLPTR_T, Defined if the currently used compiler supports C++11 @c nullptr.}