Niklas Hambüchen 57daa860e8 autoconf: Fix C++17 detection not working on Ubuntu 16.04. ... And probably many other distributions. Until now, ./configure would fail silently printing a warning ./configure: line 4621: AX_CXX_COMPILE_STDCXX_17: command not found and then continuing, later failing with a C++ #error saying that some C++11 feature isn't supported (it didn't even get to the C++17 features). This is because older distributions don't come with the `AX_CXX_COMPILE_STDCXX_17` m4 macro. This commit vendors that macro accordingly. Now ./configure complains correctly: configure: error: *** A compiler with support for C++17 language features is required. On Ubuntu 16.04, ./configure completes if a newer compiler is used, e.g. with gcc-7 from https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test using: ./bootstrap.sh ./configure CXX=g++-7 --disable-doc-gen --with-boost=$(nix-build --no-link '<nixpkgs>' -A boost.dev)		2019-07-03 04:32:25 +02:00
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ax_cxx_compile_stdcxx.m4
ax_cxx_compile_stdcxx_17.m4