__GLIBC_USE_IEC_60559_FUNCS_EXT_C2X

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
extern (C)
static if(!is(typeof(__GLIBC_USE_IEC_60559_FUNCS_EXT_C2X)))
enum __GLIBC_USE_IEC_60559_FUNCS_EXT_C2X = 1;

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