#2332 Include wcslen() declaration

[Vadim Zeitlin]

A AC_CHECK_FUNCS(wcsnlen) configure checks succeeds because wcsnlen() is found in the library, but the compilation of the (C++) code using it fails because there is no declaration for it in MinGW 5.3.0 headers. Even though this can, of course, be worked around, it would be nice if it could be included, as it's already done for strnlen().

[Keith Marshall]

This isn't as trivial a change as you might expect; nonetheless, I committed [a0d99f], adapting the existing strnlen() implementation to also provide wcsnlen(), and [d7c921] to make the two functions declaratively consistent.