"Redefinition of a extern inline not supported in C99" build failure(s) — clang 8.0.1

clang refuses this code while gcc accepts it:
// foo.c:5:6: error: redefinition of a 'extern inline' function 'foo' is not
//      supported in C99 mode
extern inline void foo ()
{
}

void foo ()
{
}

No error detected for version 8.0.1