MFC r263998:

Rename __wchar_t so it no longer conflicts with __wchar_t from clang 3.4
-fms-extensions.

(cherry picked from commit ab93c8e66ba4729148bd26479a2452dc4b9ee56b)

This is a workaround for a NanoBSD or FreeBSD issue that prevents cross-building
FreeBSD 9.2 on a FreeBSD 9.3 host system.

Ticket: #6024