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

[sjames at gcc dot gnu.org]

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

--- Comment #2 from Sam James <sjames at gcc dot gnu.org> ---
Interestingly, in binutils, the source versions are lowercase, but some
translations have the msgid as W.

The case changed in:

commit 9793eb77929a4ab2c0192d9bf5d3f8d20dd17394
Author:     Alan Modra <amodra@gmail.com>
AuthorDate: Tue Feb 20 19:32:12 2018 +1030
Commit:     Alan Modra <amodra@gmail.com>
CommitDate: Mon Feb 26 09:29:15 2018 +1030

    ELF linker messages

    This fixes a number of cases where we capitalized error messages or
    printed a full-stop, and corrects "Dwarf" to "DWARF".

I think we probably want to accept both.