Eliminate unused variable warnings for conditionally compiled code. We had more or less decided not to bother trying to eliminate all these warnings for cases in which external dependencies are missing. But then we get people trying to fix these in various ways, so we might as well do it for all cases and use a consistent method. * oct-conf-post.in.h (octave_unused_parameter): New function for C++ code and new macro for C code. * mk-octave-config-h.sh: Emit octave_unused_parameter function and macro for octave-config.h. * CSparse.cc, __delaunayn__.cc, __eigs__.cc, __fltk_uigetfile__.cc, __glpk__.cc, __magick_read__.cc, __osmesa_print__.cc, __voronoi__.cc, amd.cc, audiodevinfo.cc, audioread.cc, ccolamd.cc, cdisplay.c, colamd.cc, convhulln.cc, dSparse.cc, dmperm.cc, fftw.cc, gl-render.cc, lo-error.c, load-save.cc, ls-hdf5.cc, ls-mat5.cc, oct-hdf5-types.cc, ov-base-int.cc, ov-bool-mat.cc, ov-bool-sparse.cc, ov-bool.cc, ov-cell.cc, ov-class.cc, ov-complex.cc, ov-cx-mat.cc, ov-cx-sparse.cc, ov-fcn-handle.cc, ov-fcn-inline.cc, ov-float.cc, ov-flt-complex.cc, ov-flt-cx-mat.cc, ov-flt-re-mat.cc, ov-java.cc, ov-range.cc, ov-re-mat.cc, ov-re-sparse.cc, ov-scalar.cc, ov-str-mat.cc, ov-struct.cc, sparse-chol.cc, sparse-dmsolve.cc, sparse-lu.cc, sparse-qr.cc, sparse-util.cc, symbfact.cc: Use octave_unused_parameter to eliminate warnings for conditionally compiled code.

--- a/libinterp/dldfcn/convhulln.cc	Fri May 13 09:27:38 2016 -0400
+++ b/libinterp/dldfcn/convhulln.cc	Fri May 13 09:36:14 2016 -0400
@@ -44,11 +44,12 @@
 #include "unwind-prot.h"
 
 #if defined (HAVE_QHULL)
+
 #  include "oct-qhull.h"
+
 #  if defined (NEED_QHULL_VERSION)
 char qh_version[] = "convhulln.oct 2007-07-24";
 #  endif
-#endif
 
 static void
 close_fcn (FILE *f)
@@ -70,6 +71,8 @@
   return true;
 }
 
+#endif
+
 DEFUN_DLD (convhulln, args, nargout,
            "-*- texinfo -*-\n\
 @deftypefn  {} {@var{h} =} convhulln (@var{pts})\n\
@@ -290,7 +293,12 @@
   return retval;
 
 #else
+
+  octave_unused_parameter (args);
+  octave_unused_parameter (nargout);
+
   err_disabled_feature ("convhulln", "Qhull");
+
 #endif
 }