Define MY_DLL_BUILDING symbol for dll sample DLL build.

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