gnu: windowmaker: Leave 'wmaker' unwrapped.

Fixes <http://bugs.gnu.org/18698>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/packages/gnustep.scm (windowmaker)[arguments]: Add 'pre-configure'
  phase.  Change 'wrap' phase to wrap 'wmaker.inst', not 'wmaker'.