syntax error caused by unconditional `__has_builtin` usage

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Encountered while working on #438.

#if __has_builtin(__builtin_bit_cast)
#endif

a.cpp:1: syntax error: failed to evaluate #if condition, undefined function-like macro invocation: __has_builtin( ... )

See https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fbuiltin.html.