Fix compilation with wx built with --enable-stl

--enable-stl replaces the implicit conversion to char/wchar_t* with one
to std::[w]string. This breaks conversions to boost::filesytem::path and
makes some ternaries ambiguous (which GCC helpfully reports as no
conversion existing rather than an ambiguous conversion).
Fix directory iteration on Vista

FindExInfoBasic was added in Windows 7, not Vista.