[CMake] add_custom_command and DEPENDS

[Talin]

After several years of working with CMake, I have to say that I still do not
understand the behavior of the DEPENDS clause in add_custom_command.

The main problem is a fairly simple one: I have an add_custom_command in one
subdirectory, the output of which is needed by an add_custom_command in
another subdirectory. Since add_custom_command can't refer to targets in
other directories, I have followed the advice of various experts (on this
list and on stackoverflow) and made additional targets via
add_custom_target, in both subdirectories.

So in directory A, I have an add_custom_command which generates a file, and
then I have an add_custom_target which depends on that file. In directory B,
I have another custom target which uses add_dependencies to add a dependency
to the custom target defined in A. Finally, I have an add_custom_command
which has a DEPENDS clause adding a dependency to the target defined in that
directory.

It's the last step that doesn't work - it says "No rule to make target
<targetname>, needed by <filename>.  Stop." Even though <targetname> is
defined earlier in the same CMakeLists.txt file.

According to my reading of the CMake documentation, this should work
(although the docs are quite confusing on this point). Here's what the doc
says about the DEPENDS clause of add_custom_command: "If DEPENDS specifies
any target (created by an ADD_* command) a target-level dependency is
created to make sure the target is built before any target using this custom
command." I'm assuming that the words "specifies any target" means that you
can pass the name of a target as an argument to DEPENDS. If that's not what
it means, please tell me.

Caveat: I'm still working with 2.8.3 - I see that generator expressions in
2.8.4 would solve my problems, but at the moment I'm stuck with the version
that Ubuntu distributes.

In any case, I've been struggling with this specific problem off and on for
over 3 years, asking for help on various forums, and I've never found an
answer that actually works for me.

-- 
-- Talin
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