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)

doc/manual/installation/prerequisites-source.xml

@@ -13,7 +13,7 @@
   <listitem><para>Bash Shell. The <literal>./configure</literal> script
   relies on bashisms, so Bash is required.</para></listitem>
 
-  <listitem><para>A version of GCC or Clang that supports C++14.</para></listitem>
+  <listitem><para>A version of GCC or Clang that supports C++17.</para></listitem>
 
   <listitem><para><command>pkg-config</command> to locate
   dependencies.  If your distribution does not provide it, you can get