These tests are very tricky - in general -fshort-wchar is a dangerous
option because, since the standard library is not compiled with it, you
can't use any functions from either C or C++ standard library without
getting ABI mismatch.
The reason we want to use this is to do coverage testing on UTF32->UTF16
and UTF16->UTF32 conversion paths, that generally aren't hit on
gcc/clang. To do this, we carefully work around any internal calls to
wcslen/wcscmp that pugixml might be doing and don't use any wstring
functions.
