__has_c_attribute ¶The special operator __has_c_attribute (operand) may be
used in ‘#if’ and ‘#elif’ expressions in C code to test
whether the attribute referenced by its operand is recognized by
GCC in attributes using the ‘[[]]’ syntax. GNU attributes must
be specified with the scope ‘gnu’ or ‘__gnu__’ with
__has_c_attribute. When operand designates a supported
standard attribute it evaluates to an integer constant of the form
YYYYMM indicating the year and month when the attribute was
first introduced into the C standard, or when the syntax of operands
to the attribute was extended in the C standard.