syntax error caused by unconditional `__has_cpp_attribute` usage

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Encountered while working on #438.

# if __has_cpp_attribute(maybe_unused)
# endif

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

See https://en.cppreference.com/w/cpp/feature_test.html#Attributes https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005fcpp_005fattribute.html