[libcxx] [test] [ctime] Ignore -Wformat-zero-length warnings

[mgorny]

Summary

Explicitly disable the -Wformat-zero-length diagnostic when running
ctime tests, since one of the test cases passes zero-length format
string to strftime(). When strftime() is appropriately decorated
with attribute(format, ...), this caused the test to fail because
of this warning (e.g. on NetBSD).

[krytarowski]

Jut asking, do you intend to cover GCC and Clang with this check?

[mgorny]

In D55661#1330181, @krytarowski wrote:

Jut asking, do you intend to cover GCC and Clang with this check?

Yes, I've tested it and it makes the test pass on both.

test/std/utilities/time/date.time/ctime.pass.cpp

 #endif
 
 #if TEST_STD_VER > 14 && defined(TEST_HAS_C11_FEATURES)
 #ifndef TIME_UTC
 #error TIME_UTC not defined
 #endif
 #endif
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wformat-zero-length"
+#endif
+
 int main()
 {
     std::clock_t c = 0;
     std::size_t s = 0;
     std::time_t t = 0;
     std::tm tm = {};
     char str[3];
     ((void)c); // Prevent unused warning