CPack: Add support for CPACK_PRODUCTBULID_RESOURCES_DIR

This allows you to copy custom files (e.g. a background image) into the
macOS installer.

Fixes: #16604

Help/release/dev/productbuild_resources.rst

@@ -0,0 +1,7 @@
+productbuild_resources
+----------------------
+
+* The :module:`CPackProductBuild` module gained a new
+  :variable:`CPACK_PRODUCTBUILD_RESOURCES_DIR` variable to
+  specify resources to be copied into the ``Resources``
+  directory.

Modules/CPackProductBuild.cmake

@@ -46,3 +46,11 @@
 #
 #  Specify a specific keychain to search for the signing identity.
 #
+#
+# .. variable:: CPACK_PRODUCTBUILD_RESOURCES_DIR
+#
+#  If specified the productbuild generator copies files from this directory
+#  (including subdirectories) to the ``Resources`` directory. This is done
+#  before the :variable:`CPACK_RESOURCE_FILE_WELCOME`,
+#  :variable:`CPACK_RESOURCE_FILE_README`, and
+#  :variable:`CPACK_RESOURCE_FILE_LICENSE` files are copied.

Source/CPack/cmCPackProductBuildGenerator.cxx

@@ -59,12 +59,25 @@ int cmCPackProductBuildGenerator::PackageFiles()
     }
   }
 
-  // Copy or create all of the resource files we need.
   std::string resDir = packageDirFileName + "/Contents";
+
+  if (this->IsSet("CPACK_PRODUCTBUILD_RESOURCES_DIR")) {
+    std::string userResDir =
+      this->GetOption("CPACK_PRODUCTBUILD_RESOURCES_DIR");
+
+    if (!cmSystemTools::CopyADirectory(userResDir, resDir)) {
+      cmCPackLogger(cmCPackLog::LOG_ERROR, "Problem copying the resource files"
+                      << std::endl);
+      return 0;
+    }
+  }
+
+  // Copy or create all of the resource files we need.
   if (!this->CopyCreateResourceFile("License", resDir) ||
       !this->CopyCreateResourceFile("ReadMe", resDir) ||
       !this->CopyCreateResourceFile("Welcome", resDir)) {
-    cmCPackLogger(cmCPackLog::LOG_ERROR, "Problem copying the resource files"
+    cmCPackLogger(cmCPackLog::LOG_ERROR,
+                  "Problem copying the License, ReadMe and Welcome files"
                     << std::endl);
     return 0;
   }