elf-header: Init at <libc version>

A little shim derivation to get this header for Darwin, where it is
needed for cross compilation.

There's no real reason to do glibc and musl like that, but as I'm
maintaining it I suppose I can go overboard like that.

pkgs/development/libraries/elf-header/default.nix

@@ -0,0 +1,43 @@
+{ stdenvNoCC, lib, glibc, musl }:
+
+let
+   libc =
+     if stdenvNoCC.targetPlatform.isMusl
+     then musl
+     else glibc;
+   headerPath =
+     if stdenvNoCC.targetPlatform.isMusl
+     then "musl-${libc.version}/include/elf.h"
+     else "glibc-${libc.version}/elf/elf.h";
+in
+
+stdenvNoCC.mkDerivation {
+  name = "elf-header";
+  inherit (libc) version;
+
+  src = null;
+
+  unpackPhase = "true";
+
+  dontBuild = true;
+
+  installPhase = ''
+    mkdir -p "$out/include";
+    tar -xf \
+        ${lib.escapeShellArg libc.src} \
+        ${lib.escapeShellArg headerPath} \
+        --to-stdout \
+      | sed -e '/features\.h/d' \
+      > "$out/include/elf.h"
+  '';
+
+  meta = libc.meta // {
+    description = "The datastructures of ELF according to the target platform's libc";
+    longDescription = ''
+	  The Executable and Linkable Format (ELF, formerly named Extensible Linking
+	  Format), is usually defined in a header like this.
+	'';
+    platforms = lib.platforms.all;
+    maintainers = [ lib.maintainers.ericson2314 ];
+  };
+}

pkgs/top-level/all-packages.nix

@@ -9513,6 +9513,14 @@ with pkgs;
     installLocales = config.glibc.locales or false;
   };
 
+  # Provided by libc on Operating Systems that use the Extensible Linker Format.
+  elf-header =
+    if stdenv.hostPlatform.parsed.kernel.execFormat.name == "elf"
+    then null
+    else elf-header-real;
+
+  elf-header-real = callPackage ../development/libraries/elf-header { };
+
   glibc_memusage = callPackage ../development/libraries/glibc {
     installLocales = false;
     withGd = true;