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/makefile.gcc

@@ -25,8 +25,8 @@ MY_DLL_CXXFLAGS = $(__DEBUGINFO) $(__OPTIMIZEFLAG) $(__THREADSFLAG) $(GCCFLAGS)
 	$(__EXCEPTIONS_DEFINE_p) $(__RTTI_DEFINE_p) $(__THREAD_DEFINE_p) \
 	$(__UNICODE_DEFINE_p) $(__MSLU_DEFINE_p) $(__GFXCTX_DEFINE_p) \
 	-I$(SETUPHDIR) -I.\..\..\include $(____CAIRO_INCLUDEDIR_FILENAMES) -W -Wall \
-	$(__DLLFLAG_p) -I. $(__RTTIFLAG) $(__EXCEPTIONSFLAG) -Wno-ctor-dtor-privacy \
-	$(CPPFLAGS) $(CXXFLAGS)
+	$(__DLLFLAG_p) -DMY_DLL_BUILDING -I. $(__RTTIFLAG) $(__EXCEPTIONSFLAG) \
+	-Wno-ctor-dtor-privacy $(CPPFLAGS) $(CXXFLAGS)
 MY_DLL_OBJECTS =  \
 	$(OBJS)\my_dll_my_dll.o
 WX_EXE_CXXFLAGS = $(__DEBUGINFO) $(__OPTIMIZEFLAG) $(__THREADSFLAG) $(GCCFLAGS) \