Configuring gdb_wchar.h

[Eli Zaretskii]

gdb_wchar.h and the corresponding configury test are too naive: they
assume that if wchar.h exists, the full array of wide-character
functions, such as wcslen, iswprint, iswdigit, and btowc, also exist.
It needs to test for the individual functions as well, or at least for
some of them.

The actual case in point is the DJGPP build.  The DJGPP library has
wchar.h and there's a port of libiconv, but the functions gdb_wchar.h
expects for wide character support are not there.