[PATCH 4/6] libstdc++: Update AC_LANG_{SAVE,RESTORE} to AC_LANG_{PUSH,POP} [PR103459]

[Jonathan Wakely]

On Wed, 24 Jun 2026 at 22:58 -0400, Pietro Monteiro wrote:
>AC_LANG_SAVE, AC_LANG_C, AC_LANG_CPLUSPLUS, and AC_LANG_RESTORE have
>been deprecated by Autoconf.  Update with AC_LANG_PUSH and
>AC_LANG_POP.

I had a patch for this in GCC 16 stage 1 that I never pushed:
https://gcc.gnu.org/pipermail/gcc-patches/2025-April/681893.html

I'll review yours now ...