[CMake] How to generate a source file with unpredictable dependency

[jlh]

Hello!

Alexander Neundorf wrote:
> ADD_CUSTOM_TARGET(make_version_target ${CMAKE_CURRENT_SOURCE_DIR}/makeversion
> 	COMMENT "Checking version.cpp" VERBATIM)
> SET_SOURCE_FILES_PROPERTIES(version.cpp PROPERTIES GENERATED TRUE)
> 
> create your target...
> 
> ADD_EXECUTABLE(foo main.cpp version.cpp)
> ADD_DEPENDENCIES(foo make_version_target)

This seems to work, thanks.  It also helps with my concept that
version.cpp is not a target.  :)  Just there's an additional line
of output saying "Built target make_version_target", but I guess
if I can find a nicer name for the target that will be ok.

> You can skip the following line:
>> SET_SOURCE_FILES_PROPERTIES(version.cpp PROPERTIES GENERATED TRUE)

No, I can't.  cmake will error if the file doesn't exist yet:

CMake Error: Cannot find source file "/path/to/project/version.cpp"
	for target "project"

Thanks again,
jlh