gnu: wireless-tools: Build shared library; remove non-free headers.

* gnu/packages/linux.scm (wireless-tools)[source]: Replace unneeded
  old snippet with a new one that removes non-free files.
  [arguments]: Use modify-phases.  Remove custom 'configure' phase.  Add
  make-flags.
  [license]: Change to (list gpl2 lgpl2.1+).