xcbuild: refactor

This reworks some of xcbuild logic to make it more compatible with
Apple’s SDK.

- Add a fake version of xcrun & xcode-select
- Cleanup platform generation. Clang does not like having 20 char
  hashes in sysroot so it is much easier to just build the parent
  directory for each runCommand. This is a little awkward but I have
  renamed everything with an added ‘s’ to make the distinction more clear.
- Cleaned up wrapper.nix in some different ways
- Reuse some versioning logic so that we don’t end up with two
  different versions of Xcode or SDK reported.

pkgs/development/tools/xcbuild/toolchains.nix

@@ -0,0 +1,71 @@
+{ runCommand, toolchainName, fetchurl, makeWrapper, stdenv
+, buildPackages, lib, writeText }:
+
+let
+
+  inherit (lib) getBin optionalString;
+  inherit (lib.generators) toPlist;
+
+  ToolchainInfo = {
+    Identifier = toolchainName;
+  };
+
+  # We could pull this out of developer_cmds but it adds an annoying
+  # loop if we want to bootstrap and this is just a tiny script so I'm
+  # not going to bother.
+  mkdep-darwin-src = fetchurl {
+    url        = "https://opensource.apple.com/source/developer_cmds/developer_cmds-63/mkdep/mkdep.sh";
+    sha256     = "0n4wpqfslfjs5zbys5yri8pfi2awyhlmknsf6laa5jzqbzq9x541";
+    executable = true;
+  };
+in
+
+runCommand "Toolchains" {
+  nativeBuildInputs = [ makeWrapper ];
+} (''
+  toolchain=$out/XcodeDefault.xctoolchain
+  mkdir -p $toolchain
+
+  install -D ${writeText "ToolchainInfo.plist" (toPlist {} ToolchainInfo)} $toolchain/ToolchainInfo.plist
+
+  mkdir -p $toolchain/usr/include
+  mkdir -p $toolchain/usr/lib
+  mkdir -p $toolchain/usr/libexec
+  mkdir -p $toolchain/usr/share
+  mkdir -p $toolchain/usr/bin
+
+  for bin in ${getBin stdenv.cc}/bin/*; do
+    ln -s $bin $toolchain/usr/bin
+  done
+
+  for bin in ${getBin stdenv.cc.bintools.bintools}/bin/*; do
+    if ! [ -e "$toolchain/usr/bin/$(basename $bin)" ]; then
+      ln -s $bin $toolchain/usr/bin
+    fi
+  done
+
+  ln -s ${buildPackages.yacc}/bin/yacc $toolchain/usr/bin/yacc
+  ln -s ${buildPackages.yacc}/bin/bison $toolchain/usr/bin/bison
+  ln -s ${buildPackages.flex}/bin/flex $toolchain/usr/bin/flex
+  ln -s ${buildPackages.flex}/bin/flex++ $toolchain/usr/bin/flex++
+  ln -s $toolchain/bin/flex $toolchain/usr/bin/lex
+
+  ln -s ${buildPackages.m4}/bin/m4 $toolchain/usr/bin/m4
+  ln -s $toolchain/usr/bin/m4 $toolchain/usr/bin/gm4
+
+  ln -s ${buildPackages.unifdef}/bin/unifdef $toolchain/usr/bin/unifdef
+  ln -s ${buildPackages.unifdef}/bin/unifdefall $toolchain/usr/bin/unifdefall
+
+  ln -s ${buildPackages.gperf}/bin/gperf $toolchain/usr/bin/gperf
+  ln -s ${buildPackages.indent}/bin/indent $toolchain/usr/bin/indent
+  ln -s ${buildPackages.ctags}/bin/ctags $toolchain/usr/bin/ctags
+'' + optionalString stdenv.isDarwin ''
+  for bin in ${getBin buildPackages.darwin.cctools}/bin/*; do
+    if ! [ -e "$toolchain/usr/bin/$(basename $bin)" ]; then
+      ln -s $bin $toolchain/usr/bin
+    fi
+  done
+
+  ln -s ${buildPackages.darwin.bootstrap_cmds}/bin/mig $toolchain/usr/bin
+  ln -s ${mkdep-darwin-src} $toolchain/usr/bin/mkdep
+'')