cpack(DEB): Issue warning if custom script for control file is missing

Fixes: #17794

Source/CPack/cmCPackDebGenerator.cxx

@@ -396,6 +396,15 @@ bool DebGenerator::generateControlTar(std::string const& md5Filename) const
       }
 
       // if we can copy the file, it means it does exist, let's add it:
+      if (!cmsys::SystemTools::FileExists(i)) {
+        cmCPackLogger(cmCPackLog::LOG_WARNING,
+                      "Adding file to tar:" << std::endl
+                                            << "#top level directory: "
+                                            << this->WorkDir << std::endl
+                                            << "#missing file: " << i
+                                            << std::endl);
+      }
+
       if (cmsys::SystemTools::CopyFileIfDifferent(i, localcopy)) {
         control_tar.Add(localcopy, this->WorkDir.length(), ".");
       }

Tests/RunCMake/CPack/tests/EXTRA/DEB-stderr.txt

@@ -0,0 +1,6 @@
+CPack Warning: Adding file to tar:
+#top level directory: .*/Tests/RunCMake/DEB.EXTRA/CPack/EXTRA-build/_CPack_Packages/Linux/DEB/extra-0.1.1-Linux/bas
+#missing file: .*/Tests/RunCMake/DEB.EXTRA/CPack/EXTRA-build/conffiles
+CPack Warning: Adding file to tar:
+#top level directory: .*/Tests/RunCMake/DEB.EXTRA/CPack/EXTRA-build/_CPack_Packages/Linux/DEB/extra-0.1.1-Linux/foo
+#missing file: .*/Tests/RunCMake/DEB.EXTRA/CPack/EXTRA-build/conffiles