b4n commented on this pull request.
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+# ============================================================================
Indeed, the one [currently on the on-line AutoConf archive](https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_11.html) doesn't match the one in the corresponding Debian 8 package. I guess I should indeed update to the latest upstream, although it's functionally the same when it's used to check for C++11 and not anything newer.