cmLocalGenerator: Factor out helper to expand custom command output paths

Source/cmLocalGenerator.h

@@ -22,6 +22,7 @@
 #include "cmProperty.h"
 #include "cmStateSnapshot.h"
 
+class cmCompiledGeneratorExpression;
 class cmComputeLinkInformation;
 class cmCustomCommandGenerator;
 class cmCustomCommandLines;
@@ -362,6 +363,9 @@ public:
     bool command_expand_lists = false, const std::string& job_pool = "",
     bool stdPipesUTF8 = false);
 
+  std::vector<std::string> ExpandCustomCommandOutputPaths(
+    cmCompiledGeneratorExpression const& cge, std::string const& config);
+
   /**
    * Add target byproducts.
    */