Define MY_DLL_BUILDING symbol for dll sample DLL build.

This is necessary to properly define MY_DLL_DECL as either WXIMPORT or
WXEXPORT depending on whether my_dll.h is included from DLL itself or from
outside it. Without this, MSVC generated warnings about inconsistent dll
linkage.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62802 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/dll/my_dll.h

@@ -15,11 +15,17 @@
 extern "C" {
 #endif
 
+#ifdef MY_DLL_BUILDING
+    #define MY_DLL_DECL WXEXPORT
+#else
+    #define MY_DLL_DECL WXIMPORT
+#endif
+
 // launch wx UI from some application that may or may not be written in wx
-WXIMPORT void run_wx_gui_from_dll(const char *title);
+MY_DLL_DECL void run_wx_gui_from_dll(const char *title);
 
 // run this to shutdown running threads etc.
-WXIMPORT void wx_dll_cleanup(void);
+MY_DLL_DECL void wx_dll_cleanup();
 
 
 #ifdef __cplusplus