Use err_disabled_feature, warn_disabled_feature throughout code base. In liboctave, use the same error text as err_disabled_feature, but call error_handler directly because err_disabled_feature is in libinterp. * errwarn.cc (err_disabled_feature): Don't print leading "%s:" if calling function is "". * errwarn.cc (warn_disabled_feature): New function. Same msg as err_disabled_feature but uses warning rather than error. * errwarn.h (warn_disabled_feature): prototype for new function. * file-io.cc, gl-render.cc, gl2ps-renderer.cc, load-save.cc, ls-mat5.cc, oct-hdf5-types.cc, pt-jit.cc, syscalls.cc, sysdep.cc, toplev.cc, __delaunayn__.cc, __eigs__.cc, __fltk_uigetfile__.cc, __init_fltk__.cc, __osmesa_print__.cc, __voronoi__.cc, amd.cc, ccolamd.cc, colamd.cc, convhulln.cc, dmperm.cc, fftw.cc, symbfact.cc: Replace calls to error about missing feature with calls to err_disabled_feature. Replace calls to warning about missing feature with calls to warn_disabled_feature. * CSparse.cc, dSparse.cc, SparseCmplxLU.cc, SparseCmplxQR.cc, SparseQR.cc, SparsedbleLU.cc, sparse-base-chol.cc, sparse-dmsolve.cc, oct-shlib.cc: Use same text of message from err_disabled_feature but call (*current_liboctave_error_handler) or (*current_liboctave_warning_with_id_handler).

--- a/libinterp/corefcn/file-io.cc	Tue Jan 19 14:38:35 2016 -0500
+++ b/libinterp/corefcn/file-io.cc	Tue Jan 19 12:44:54 2016 -0800
@@ -178,7 +178,7 @@
           use_zlib = true;
           mode.erase (pos, 1);
 #else
-          error ("this version of Octave does not support gzipped files");
+          err_disabled_feature ("", "gzipped files (zlib)");
 #endif
         }