[tz] static_assert macro

[Christos Zoulas via tz]

Hello,

The new tzcode uses static_assert() with no message which is a c2x 
extension. clang does not like that by default (without specifying 
-std=gnu2x).

/home/source/ab/HEAD-llvm/src/lib/libc/time/private.h:1017:52: error: 
'_Static_assert' with no message is a C2x extension 
[-Werror,-Wc2x-extensions]
              || TIME_T_MAX >> (TYPE_BIT(time_t) - 2) == 1);
                                                          ^
                                                          , ""

Best,

christos