[Bug libstdc++/126452] Typo in check for --gc-sections support in acinclude.m4

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126452

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That's not right because m4 replaces [Ww] with just Ww