cmMakefile: Extract utilities used for creation of custom commands

Decompose creation of custom commands further into logical steps.

Source/cmCustomCommandTypes.h

@@ -5,6 +5,8 @@
 
 #include "cmConfigure.h" // IWYU pragma: keep
 
+#include <string>
+
 /** Target custom command type */
 enum class cmCustomCommandType
 {
@@ -27,4 +29,11 @@ enum class cmObjectLibraryCommands
   Accept
 };
 
+/** Utility target output source file name.  */
+struct cmUtilityOutput
+{
+  std::string Name;
+  std::string NameCMP0049;
+};
+
 #endif