[CMake] CmakeLists chain cannot find source added with add_custom_command

[J Decker]

I have this chain of makefiles.  It adds a custom rule to build a source
file, and then a subdirectory cannot reference that file.

The first is added custom rule for M:/tmp/cmake-chain/sub/src/sack.c
and then later cannot find M:/tmp/cmake-chain/sub/src/sack.c
though it's really referencing it
as M:/tmp/cmake-chain/sub/sub2/../src/sack.c

tested with 3.6.0-rc4 and 3.9.0-rc5

if the add_custom_rule and add_executable are in the same makefile even if
there's a added path to make it not exactly the same source as the
'referencing source' message logs; it works.


--- CMakeLists.txt ---
cmake_minimum_required(VERSION 2.8)

message( "Adding custom rule for ${CMAKE_CURRENT_SOURCE_DIR}/src/sack.c" )
  add_custom_command( OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/src/sack.c
  COMMAND cmake -E echo do something to make src/sack.c
  )
add_subdirectory( sub2 )
-------------------------------------

--- sub2/CmakeLists.txt ---
message( "referencing source ${CMAKE_CURRENT_SOURCE_DIR}/../src/sack.c" )
add_executable( something ${CMAKE_CURRENT_SOURCE_DIR}/../src/sack.c )

-------------------------------------

--- Truncated output log ------
Adding custom rule for M:/tmp/cmake-chain/sub/src/sack.c
referencing source M:/tmp/cmake-chain/sub/sub2/../src/sack.c
-- Configuring done
CMake Error at sub2/CMakeLists.txt:3 (add_executable):
  Cannot find source file:

    M:/tmp/cmake-chain/sub/src/sack.c

  Tried extensions .c .C .c++ .cc .cpp .cxx .m .M .mm .h .hh .h++ .hm .hpp
  .hxx .in .txx
--------------------------------------


------ Flat CmakeLists.txt which works ---------------
cmake_minimum_required(VERSION 2.8)

message( "Adding custom rule for ${CMAKE_CURRENT_SOURCE_DIR}/src/sack.c" )
  add_custom_command( OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/src/sack.c
COMMAND cmake -e echo make file.
  )

message( "referencing source ${CMAKE_CURRENT_SOURCE_DIR}/src/sack.c" )
add_executable( something ${CMAKE_CURRENT_SOURCE_DIR}/sub2/../src/sack.c )
--------------------------------------

----- flat cmakelists output.... -----------
Adding custom rule for M:/tmp/cmake-chain-flat/src/sack.c
referencing source M:/tmp/cmake-chain-flat/sub2/../src/sack.c
-- Configuring done
--------------------------------------
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