Use wgetcwd() with MinGW to handle non-ASCII working directory

All versions of MinGW seem to have this function, so use it to ensure that
wxGetCwd() returns the correct result even when the current directory contains
non-ASCII characters.

Closes https://github.com/wxWidgets/wxWidgets/pull/307

src/common/filefn.cpp

@@ -82,7 +82,7 @@
 #endif
 
 // TODO: Borland probably has _wgetcwd as well?
-#ifdef _MSC_VER
+#if defined(_MSC_VER) || defined(__MINGW32__)
     #define HAVE_WGETCWD
 #endif