Skip file-level dependencies on custom targets (#11332)

A custom command may name a target created by add_custom_target in its
DEPENDS field.  Treat this case as a target-level dependency only since
a custom target provides no standard file on which to add a file-level
dependency.

Source/cmLocalGenerator.h

@@ -158,6 +158,7 @@ public:
 
   /** Translate a dependency as given in CMake code to the name to
       appear in a generated build file.  If the given name is that of
+      a utility target, returns false.  If the given name is that of
       a CMake target it will be transformed to the real output
       location of that target for the given configuration.  If the
       given name is the full path to a file it will be returned.