kernel 4.14 require libelf to compile modules.

[...]
make modules -C /nix/store/h1vzl6bq4wif3m8dd1bw2p3fv4shjg3n-linux-4.14.9-dev/lib/modules/4.14.9/build EXTRA_CFLAGS=-Werror-implicit-function-declaration M=/tmp/nix-build-spl-kernel-2017-11-16-4.14.9.drv-0/source/build
/nix/store/h1vzl6bq4wif3m8dd1bw2p3fv4shjg3n-linux-4.14.9-dev/lib/modules/4.14.9/source/Makefile:939: *** "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel". Stop.

This patch introduces kernel.moduleBuildDependencies to avoid the logic "stdenv.lib.optional (stdenv.lib.versionAtLeast kernel.version "4.14") libelf" in multiple places.

[dezgeg did some minor tweaks on top]

pkgs/applications/virtualization/virtualbox/guest-additions/default.nix

···
 
   NIX_CFLAGS_COMPILE = "-Wno-error=incompatible-pointer-types -Wno-error=implicit-function-declaration";
 
-  buildInputs = [ patchelf cdrkit makeWrapper dbus ];
+  buildInputs = [ patchelf cdrkit makeWrapper dbus ] ++ kernel.moduleBuildDependencies;
 
   installPhase = ''
     mkdir -p $out

pkgs/os-specific/linux/kernel/manual-config.nix

···
   inherit (stdenv.lib)
     hasAttr getAttr optional optionalString optionalAttrs maintainers platforms;
 
+  # Dependencies that are required to build kernel modules
+  moduleBuildDependencies = stdenv.lib.optional (stdenv.lib.versionAtLeast version "4.14") libelf;
+
   installkernel = writeTextFile { name = "installkernel"; executable=true; text = ''
     #!${stdenv.shell} -e
     mkdir -p $4
···
         (isModular || (config.isDisabled "FIRMWARE_IN_KERNEL"));
     in (optionalAttrs isModular { outputs = [ "out" "dev" ]; }) // {
       passthru = {
-        inherit version modDirVersion config kernelPatches configfile;
+        inherit version modDirVersion config kernelPatches configfile moduleBuildDependencies;
       };
 
       inherit src;

pkgs/os-specific/linux/spl/default.nix

···
 
       patches = [ ./const.patch ./install_prefix.patch ];
 
-      nativeBuildInputs = [ autoreconfHook ];
+      nativeBuildInputs = [ autoreconfHook ] ++ optional (kernel != null) kernel.moduleBuildDependencies;
 
       hardeningDisable = [ "pic" ];
 

pkgs/os-specific/linux/sysdig/default.nix

···
 
   buildInputs = [
     cmake zlib luajit ncurses perl jsoncpp libb64 openssl curl jq gcc
-  ];
+  ] ++ optional (kernel != null) kernel.moduleBuildDependencies;
 
   hardeningDisable = [ "pic" ];
 

pkgs/os-specific/linux/tp_smapi/default.nix

···
     name = "tp-smapi-${version}";
   };
 
+  nativeBuildInputs = kernel.moduleBuildDependencies;
+
   hardeningDisable = [ "pic" ];
 
   makeFlags = [

pkgs/os-specific/linux/virtualbox/default.nix

···
     "fortify" "pic" "stackprotector"
   ];
 
+  nativeBuildInputs = kernel.moduleBuildDependencies;
+
   patches = [
     ./fix_kerndir.patch
     ./fix_kbuild.patch

pkgs/os-specific/linux/zfs/default.nix

···
 
       patches = extraPatches;
 
-      nativeBuildInputs = [ autoreconfHook nukeReferences ];
+      nativeBuildInputs = [ autoreconfHook nukeReferences ]
+         ++ optional (kernel != null) kernel.moduleBuildDependencies;
       buildInputs =
            optionals buildKernel [ spl ]
         ++ optionals buildUser [ zlib libuuid python attr ]