Fix link errors under Cygwin with wxUSE_GRAPHICS_CONTEXT==1.

Define ourselves _GdipStringFormatCachedGenericTypographic variable referenced
by Cygwin GDI+ headers to fix linking errors due to the fact that we don't
link with gdiplus.lib at all but load gdiplus.dll dynamically.

Closes #11716.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66787 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/gdiplus.cpp

@@ -734,6 +734,16 @@ wxFOR_ALL_GDIPLUS_STATUS_FUNCS(wxDECL_GDIPLUS_FUNC_TYPE)
 
 } // extern "C"
 
+// Special hack for Cygwin: its headers reference this variable which is
+// normally defined in Cygwin-specific gdiplus.lib but as we don't link with it
+// and load gdiplus.dll dynamically, it's not defined in our case resulting in
+// linking errors -- so just provide it ourselves, it doesn't matter where it
+// is and if Cygwin headers are modified to not use it in the future, it's not
+// a big deal neither, we'll just have an unused pointer.
+#ifdef __CYGWIN__
+extern "C" void *_GdipStringFormatCachedGenericTypographic = NULL;
+#endif // __CYGWIN__
+
 // ============================================================================
 // wxGdiPlus helper class
 // ============================================================================