Document the NanoSVG options

docs/doxygen/mainpages/const_wxusedef.h

@@ -177,6 +177,9 @@ library:
 @itemdef{wxUSE_MINIFRAME, Use wxMiniFrame class.}
 @itemdef{wxUSE_MOUSEWHEEL, Support mouse wheel events.}
 @itemdef{wxUSE_MSGDLG, Use wxMessageDialog class and wxMessageBox function.}
+@itemdef{wxUSE_NANOSVG, Use NanoSVG for rasterizing SVG.}
+@itemdef{wxUSE_NANOSVG_EXTERNAL, Use an external NanoSVG library when wxUSE_NANOSVG is enabled.}
+@itemdef{wxUSE_NANOSVG_EXTERNAL_ENABLE_IMPL, When the external NanoSVG library is header only, define this to enable its implementation.}
 @itemdef{wxUSE_NATIVE_DATAVIEWCTRL, Use native wxDataViewCtrl class (this option is new since wxWidgets 3.1.4).}
 @itemdef{wxUSE_NATIVE_STATUSBAR, Use native wxStatusBar class.}
 @itemdef{wxUSE_NOTEBOOK, Use wxNotebook and related classes.}