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]