[libcxx] [test] Define _CRT_STDIO_ISO_WIDE_SPECIFIERS while building tests

[mstorsjo]

Summary

This matches how libc++ itself is built. This avoids errors due to mismatch if linking libc++ statically.

libcxx/utils/libcxx/test/config.py

def configure_compile_flags(self):
         # Configure extra flags
         compile_flags_str = self.get_lit_conf('compile_flags', '')
         self.cxx.compile_flags += shlex.split(compile_flags_str)
         if self.target_info.is_windows():
             self.cxx.compile_flags += ['-D_CRT_SECURE_NO_WARNINGS']
             # Don't warn about using common but nonstandard unprefixed functions
             # like chdir, fileno.
             self.cxx.compile_flags += ['-D_CRT_NONSTDC_NO_WARNINGS']
+            # Build the tests in the same configuration as libcxx itself,
+            # to avoid mismatches if linked statically.
+            self.cxx.compile_flags += ['-D_CRT_STDIO_ISO_WIDE_SPECIFIERS']
             # Required so that tests using min/max don't fail on Windows,
             # and so that those tests don't have to be changed to tolerate
             # this insanity.
             self.cxx.compile_flags += ['-DNOMINMAX']
         additional_flags = self.get_lit_conf('test_compiler_flags')
         if additional_flags:
             self.cxx.compile_flags += shlex.split(additional_flags)