gdb/ax_cxx_compile_stdcxx.m4: upgrade

This patch upgrades gdb/ax_cxx_compile_stdcxx.m4 to follow changes
available in [1] and regenerates the configure script.

[1] https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html

Change-Id: I5b16adc65c9e48a13ad65202d58ab7a9d487214e
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>
4 files changed