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

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

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Ah so the warning we're checking for isn't just that the option is
unrecognized.

So then maybe:

--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -262,8 +262,8 @@ AC_DEFUN([GLIBCXX_CHECK_LINKER_FEATURES], [
       rm -f conftest.c
       touch conftest.c
       if $CC -c conftest.c; then
-       if $LD --gc-sections -o conftest conftest.o 2>&1 | \
-          grep "Warning: gc-sections option ignored" > /dev/null; then
+       if LC_ALL=C $LD --gc-sections -o conftest conftest.o 2>&1 | \
+          grep "[Ww]arning: gc-sections option ignored" > /dev/null; then
          ac_gcsections=no
        fi
       fi