Use strdup() even in strict ANSI mode with MinGW.

Use strdup() even in strict ANSI mode with MinGW.

MinGW headers define MSVC-compatible _strdup() even in strict ANSI mode so we
can always use it with MinGW.

This fixes a linking problem when wx itself was built without strict ANSI but
the program using it does use strict ANSI mode (e.g. due to the use of
-std=c++0x g++ option).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68025 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775