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)

1 files changed, 35 insertions, 0 deletions

diff --git a/m4/ax_cxx_compile_stdcxx_17.m4 b/m4/ax_cxx_compile_stdcxx_17.m4
new file mode 100644
index 000000000000..a6834171739b
--- /dev/null
+++ b/m4/ax_cxx_compile_stdcxx_17.m4
@@ -0,0 +1,35 @@
+# =============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_17.html
+# =============================================================================
+#
+# SYNOPSIS
+#
+#   AX_CXX_COMPILE_STDCXX_17([ext|noext], [mandatory|optional])
+#
+# DESCRIPTION
+#
+#   Check for baseline language coverage in the compiler for the C++17
+#   standard; if necessary, add switches to CXX and CXXCPP to enable
+#   support.
+#
+#   This macro is a convenience alias for calling the AX_CXX_COMPILE_STDCXX
+#   macro with the version set to C++17.  The two optional arguments are
+#   forwarded literally as the second and third argument respectively.
+#   Please see the documentation for the AX_CXX_COMPILE_STDCXX macro for
+#   more information.  If you want to use this macro, you also need to
+#   download the ax_cxx_compile_stdcxx.m4 file.
+#
+# LICENSE
+#
+#   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
+#   Copyright (c) 2016 Krzesimir Nowak <qdlacz@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved. This file is offered as-is, without any
+#   warranty.
+
+#serial 2
+
+AX_REQUIRE_DEFINED([AX_CXX_COMPILE_STDCXX])
+AC_DEFUN([AX_CXX_COMPILE_STDCXX_17], [AX_CXX_COMPILE_STDCXX([17], [$1], [$2])])