Disable the use of __thread with MinGW.

Disable the use of __thread with MinGW.

__thread keyword doesn't work correctly with at least some MinGW builds and in
particular TLS unit tests failed with the official 4.4 version.

Disable the use of __thread with MinGW entirely for now to be sure that the
generated code behaves correctly. In the future we should white list the known
good versions of MinGW and/or use run-time test for __thread support in
configure instead of compile-time one only.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65747 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775