Add ‘shell.nix’ defining Autoconf’s build dependencies in NixOS terms.

Like the existing ‘manifest.scm’, which does the same job for GNU Guix,
the point of this file is to facilitate testing Autoconf itself on NixOS:
‘nix-shell --pure shell.nix’ gives you an interactive shell with all
the necessary tools in $PATH, and nothing else.

shell.nix

@@ -0,0 +1,43 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  # included by default (since we're not using mkShellNoCC) are:
+  #    bash coreutils file findutils gawk gnugrep gnused
+  #    gnutar gzip bzip2 xz diffutils patch
+  #    binutils gcc glibc gnumake
+  packages = with pkgs;
+    # custom texliveBasic - metafont and ectt are required to build PDF documentation
+    let
+       texliveCustom = texliveBasic.withPackages (ps: [ps.metafont ps.ec]);
+    in [
+      # Additional requirements for building and running Autoconf itself
+      m4
+      perl
+
+      # Additional requirements for building from a pristine git checkout
+      automake
+      git
+      help2man
+      texinfo
+      texliveCustom
+
+      # Needed only to run the test suite comprehensively
+      # TODO: Make a variant that uses mkShellNoCC and *doesn't* install any
+      # of this and ensure that every test that probes some of them is
+      # skipped, rather than failing, when run in that environment.
+      # Currently lots of tests fail in the absence of a C compiler.
+
+      # Compilers and tools that Autoconf is capable of probing
+      bison
+      erlang
+      flex
+      gccgo
+      gfortran
+      libtool
+      # Objective-C compiler seems not to be available
+      # shtool is not available
+
+      # Needed to test AC_CHECK_LIB and AC_SEARCH_LIBS
+      zlib
+
+  ];
+}