wchar_t appears to be Unicode, but __STDC_ISO_10646__ is not defined.

Tested with both the core and the ports versions:

FreeBSD clang version 3.1 (branches/release_31 156863) 20120523
Target: x86_64-unknown-freebsd9.0
Thread model: posix

clang version 3.1 (branches/release_31)
Target: amd64-portbld-freebsd9.0
Thread model: posix

Test file:
----------
#include <stdlib.h>
#include <wchar.h>
#include <stdio.h>

int main(void)
{
#ifdef __STDC_ISO_10646__ 
    printf("__STDC_ISO_10646__ defined\n");
#else
    printf("__STDC_ISO_10646__ NOT defined\n");
#endif
    return 0;
}

Fix: 

Have the compiler define the macro to its appropriate value.
How-To-Repeat: Compile and run the above test file (here "test.c"):

$ clang test.c -o test
$ ./test
__STDC_ISO_10646__ NOT defined

$ /usr/local/bin/clang test.c -o test
$ ./test
__STDC_ISO_10646__ NOT defined