linux-headers: Improve derivation, removing cross arg

- Perl is used at build time, so must be in `nativeBuildInputs`. It's
not used at run time so it should not be in `buildInputs`, too.

- Don't treat headers like a compiler---use the build and host
platforms not host and target. Perhaps that would make sense if every
library's headers could be a separate derivation, but since that is
not feasible, best to keep the implementation and interface in the
same stage.

To do this, we used `stdenvNoCC` to get rid of the normal toolchain,
and added a dependency for the toolchain targeting the build platform
--- `buildPackages.stdenv.cc` --- thus everything is effectively slid
a stage black.

pkgs/os-specific/linux/kernel-headers/4.4.nix

···
-{ stdenv, fetchurl, perl, cross ? null }:
+{ stdenvNoCC, lib, buildPackages
+, buildPlatform, hostPlatform
+, fetchurl, perl
+}:
 
-assert cross == null -> stdenv.isLinux;
+assert hostPlatform.isLinux;
 
 let
-
   version = "4.4.10";
-
-  kernelHeadersBaseConfig =
-    if cross == null
-    then stdenv.platform.kernelHeadersBaseConfig
-    else cross.platform.kernelHeadersBaseConfig;
-
+  inherit (hostPlatform.platform) kernelHeadersBaseConfig;
 in
 
-stdenv.mkDerivation {
+stdenvNoCC.mkDerivation {
   name = "linux-headers-${version}";
 
   src = fetchurl {
···
     sha256 = "1kpjvvd9q9wwr3314q5ymvxii4dv2d27295bzly225wlc552xhja";
   };
 
-  targetConfig = if cross != null then cross.config else null;
+  targetConfig = if hostPlatform != buildPlatform then hostPlatform.config else null;
 
-  platform =
-    if cross != null then cross.platform.kernelArch else
-    if stdenv.system == "i686-linux" then "i386" else
-    if stdenv.system == "x86_64-linux" then "x86_64" else
-    if stdenv.system == "powerpc-linux" then "powerpc" else
-    if stdenv.isArm then "arm" else
-    if stdenv.platform ? kernelArch then stdenv.platform.kernelArch else
-    abort "don't know what the kernel include directory is called for this platform";
+  platform = hostPlatform.platform.kernelArch or (
+    if hostPlatform.system == "i686-linux" then "i386" else
+    if hostPlatform.system == "x86_64-linux" then "x86_64" else
+    if hostPlatform.system == "powerpc-linux" then "powerpc" else
+    if hostPlatform.isArm then "arm" else
+    abort "don't know what the kernel include directory is called for this platform");
 
-  buildInputs = [perl];
+  # It may look odd that we use `stdenvNoCC`, and yet explicit depend on a cc.
+  # We do this so we have a build->build, not build->host, C compiler.
+  nativeBuildInputs = [ buildPackages.stdenv.cc perl ];
 
-  extraIncludeDirs =
-    if cross != null then
-        (if cross.arch == "powerpc" then ["ppc"] else [])
-    else if stdenv.system == "powerpc-linux" then ["ppc"] else [];
+  extraIncludeDirs = lib.optional hostPlatform.isPowerPC ["ppc"];
 
   buildPhase = ''
     if test -n "$targetConfig"; then
···
     fi
   '';
 
-  meta = with stdenv.lib; {
+  meta = with lib; {
     description = "Header files and scripts for Linux kernel";
     license = licenses.gpl2;
     platforms = platforms.linux;

pkgs/top-level/all-packages.nix

···
 
   lkl = callPackage ../applications/virtualization/lkl { };
 
-  linuxHeaders_4_4 = callPackage ../os-specific/linux/kernel-headers/4.4.nix {
-    cross = if targetPlatform != hostPlatform then targetPlatform else null;
-  };
+  linuxHeaders_4_4 = callPackage ../os-specific/linux/kernel-headers/4.4.nix { };
   linuxHeaders = linuxHeaders_4_4;
 
   kernelPatches = callPackage ../os-specific/linux/kernel/patches.nix { };
···
   uclibc = callPackage ../os-specific/linux/uclibc { };
 
   uclibcCross = lowPrio (callPackage ../os-specific/linux/uclibc {
-    inherit (buildPackages) linuxHeaders;
     gccCross = gccCrossStageStatic;
     cross = assert targetPlatform != buildPlatform; targetPlatform;
   });