https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115311
--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> --- This definitely wouldn't work for glibc; we freely use -fno-builtin-<func> when e.g. aliasing double and long double functions when the formats are the same, where GCC gives an error if it has the functions as built-in and so objects to the incompatible types, and we don't limit this to the functions GCC has as built-in in a particular version but also use those options for functions that might be built-in in future (e.g. C23 functions). The number of functions involved is large, needing configure tests for all of them would be a bad idea. That is, it's useful to use -fno-builtin-<func> for a valid function name that might be built-in in future without needing configure tests for whether it's built-in in a particular version.
