Don't redefine _FORCENAMELESSUNION it if it is already defined. Fixes a warning when compiling with MinGW.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76697 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775