clang -O2  -Wall -Wextra -Werror=implicit-function-declaration wchar/wcsnlen.c -o wchar/wcsnlen -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
wchar/wcsnlen.c:9:7: error: implicit declaration of function 'wcsnlen' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( wcsnlen(L"foo", 2) != 2 )
             ^
wchar/wcsnlen.c:9:7: note: did you mean 'wcslen'?
/usr/include/wchar.h:115:8: note: 'wcslen' declared here
size_t  wcslen(const wchar_t *);
        ^
1 error generated.