[Bug c/126702] undefined reference to inline function

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126702

--- Comment #5 from Sam James <sjames at gcc dot gnu.org> ---
i.e. 'inline' means no out-of-line definition will be emitted. It doesn't mean
the compiler must actually inline it (as there may be another copy). At say,
-O0, no inlining is done, so you're left with an undefined reference.

You can use -fgnu89-inline if you want.