dpkg-shlibdeps: Add a hint to the error on no dependency information found

Suggest in the output to check if the library is actually packaged.

[guillem@debian.org:
 - Line-wrap to 80 columns.
 - Slight rewording. ]

Closes: #756230

Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -79,6 +79,9 @@ dpkg (1.17.11) UNRELEASED; urgency=low
   * Warn on usage of deprecated Source-Version substvar.
   * Say OpenPGP instead of PGP when referring to the standard on code comments
     and output messages.
+  * Add a hint to the “no dependency information found” error message in
+    dpkg-shlibdeps, to check if the library is actually packaged.
+    Thanks to Sylvestre Ledru <sylvestre@debian.org>. Closes: #756230
 
   [ Updated programs translations ]
   * Danish (Joe Dalton). Closes: #754127

scripts/dpkg-shlibdeps.pl

@@ -291,7 +291,9 @@ foreach my $file (keys %exec) {
 		    # 3/ when we have been asked to do so
 		    $ignore++ if $ignore_missing_info;
 		    error(_g('no dependency information found for %s ' .
-		             '(used by %s)'), $lib, $file)
+		             '(used by %s)\n' .
+		             'Hint: check if the library actually comes ' .
+		             'from a package.'), $lib, $file)
 		        unless $ignore;
 		}
 	    }