[CMake] Need help with dependency of generated header file

[j s]

I'm trying to port a legacy project to using cmake.  The header files
are needed by the sources.
The header files are generated so that the source file can compile.

The header files are regenerated if its dependency is changed.  This
doesn't cause the source file to be rebuilt, even though it depends on
these header files.

I'm using cmake 2.8.10


SET(SRCS
foo.c
)

ADD_CUSTOM_COMMAND(
  OUTPUT ${CMAKE_CURRENT_SOURCE_DIR}/foomsgId.h
${CMAKE_CURRENT_SOURCE_DIR}/foomsgDef.h
  WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
  COMMAND ${MSG_CONVERT}
  ARGS foomessages.txt
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/foomessages.txt
)

ADD_CUSTOM_TARGET(
  generate_foo_messages
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/foomsgId.h
${CMAKE_CURRENT_SOURCE_DIR}/foomsgDef.h
)

ADD_LIBRARY(foo SHARED ${SRCS})
SET_TARGET_PROPERTIES(foo PROPERTIES PREFIX "" SUFFIX ".so" OUTPUT_NAME "foo")
ADD_DEPENDENCIES(foo generate_foo_messages)