Convert button clicked event handler from member function to extern "C" free standing function to try to fix linux segfault(#21).
Fix test crash on Linux.

 * The wxMessageBox ctor's style argument must contain one of wxOK, wxYES or wxYES_NO.
Fix test crash on Linux.

 * The wxMessageBox ctor's style argument must contain one of wxOK, wxYES or wxYES_NO.