Define wxHAS_NULLPTR_T if nullptr is supported

This is necessary because MSVS versions as old as 2010 support it, even
if they don't have full C++11 support and so don't define __cplusplus
appropriately.

Extract the existing check for nullptr from wx/strvararg.h to wx/defs.h
in order to allow reusing it.

docs/doxygen/mainpages/const_cpp.h

@@ -187,6 +187,7 @@ Currently the following symbols exist:
 @itemdef{wxHAS_MULTIPLE_FILEDLG_FILTERS, Defined if wxFileDialog supports multiple ('|'-separated) filters.}
 @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.}
 @itemdef{wxHAS_IMAGES_IN_RESOURCES, Defined if <a href="http://en.wikipedia.org/wiki/Resource_(Windows)">
     Windows resource files</a> or OS/2 resource files are available on the current platform.}
 @itemdef{wxHAS_POWER_EVENTS, Defined if wxPowerEvent are ever generated on the current platform.}