Add hook function to handle missing Octave components * __gripe_missing_component__.m: New internal function to handle errors due to missing Octave components. * scripts/help/module.mk (help_FCN_FILES): Add __gripe_missing_component__.m. * variables.cc (Vmissing_component_hook): New internal variable. * doc.m, mkoctfile.m, configure_make.m: Call __gripe_missing_component__ if required files are not found. * NEWS: Mention missing_component_hook.

--- a/scripts/help/doc.m
+++ b/scripts/help/doc.m
@@ -79,6 +79,10 @@
 
       if (err < 0)
         info_file_name = info_file ();
+
+        if (! exist (info_file_name, "file"))
+          __gripe_missing_component__ ("doc", "info-file");
+        endif
       endif
 
       ## FIXME -- don't change the order of the arguments below because