lighttpd: fix build warnings

The build complains about missing "file" and "which" commands, so add them as
build inputs.

"file" is used by the autotools configure script to tweak what -m flag
(if any) to pass to the linker when it asks it for shared library
support.

Here is an example of -m values for GNU ld:
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   i386linux
   elf_l1om
   elf_k1om

"which" is used in the build phase to look for svnversion and git, to build a
version stamp. Since we build from a release tarball (and don't pass svn or git
as inputs either), this check fails and falls back to the version number in the
tarball.

There is one build warning left, but I think this is normal on NixOS:

  /tmp/nix-build-lighttpd-1.4.32.drv-0/lighttpd-1.4.32/libtool: line 1085: ldconfig: command not found