Be like stdlib.h and bring in a wchar_t definition and use it to prototype

Be like stdlib.h and bring in a wchar_t definition and use it to prototype
wcstoimax and wcstoumax, rather than spelling it __wchar_t.  This is necessary
to use these functions in C++ where wchar_t is different to __wchar_t and is
a built-in type.

It may be better to use __wchar_t here and to simply define __wchar_t as being
wchar_t in C++ mode rather than to bring in wchar_t, but this is less invasive
and follows our existing practice, and restores wchar_t usage in this file to
what it was before r1.8.