[CMake] add_custom_command and generated true

[Micha Renner]

Recently, I learned that ADD_CUSTOM_COMMAND generated the property
GENERATED TRUE for the generated file.

Somehow I have trouble in this situation:

In a top level CMakeLists file a source file, blue.c, is generated. This
file should be used in a subdirectory with its own CMakeLists file.

Running CMake generates the message: "cannot find blue.c etc..."

Okay, the information, that this is a generated sourcefile is, maybe,
lost on the transition to the subdirectory.

So, I inserted in the CMakeLists file of subdirectory the line:
	SET_SOURCE_FILES_PROPERTIES(blue.c PROPERTIES GENERATED TRUE)
which generates the make message
	No rule to build target subdirectory/blue.c

I changed the inserted line (see above) to...
	SET_SOURCE_FILES_PROPERTIES(${CMAKE_CURRENT_SOURCE_DIR}/blue.c
		 PROPERTIES GENERATED TRUE) 
... which is okay.

=> Two question

- Is it general true, that properties are lost on transition to a
  subdirectory?
- In which cases is it absolute necessary to work with absolute paths
  and SET_SOURCE_FILES?

In the appendix is a small example (Unix only)

Greetings

Micha



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