[CMake] [BUG] add_custom_command(TARGET ...) can't see in scope target

[Dan Liew]

Hi,

I'm almost certain this is a bug but I thought I'd ask the mailing lists first.

I'm observing a problem with CMake's add_custom_command() when using
the TARGET version of the signature.

Code demonstrating the issue can be found at [1]

In ``test/CMakeLists.txt`` [2] I try using add_custom_command() to
build and run an executable every time the ``foolib`` library is
rebuilt. The motivation behind doing this is to have some very quick
and simple library tests run every time the library is built.

This use of add_custom_command() does not seem to work at all.

Using CMake 2.8.12.2 the requested command is not invoked when running
``make foolib``. I am aware that the custom command won't run if
``foolib`` has already been made but even if I do a clean build the
custom command specified does not run.

When using CMake 3.2.3 I get a warning about CMP0040 which complains
that the target passed to the TARGET argument in
``add_custom_command()`` doesn't exist.

```
  Policy CMP0040 is not set: The target in the TARGET signature of
  add_custom_command() must exist.  Run "cmake --help-policy CMP0040" for
  policy details.  Use the cmake_policy command to set the policy and
  suppress this warning.

  The target name "foolib" is unknown in this context.
This warning is for project developers.  Use -Wno-dev to suppress it
```


This doesn't make sense the target **clearly exists and is in scope**
because the ``simple_test`` executable links against it and also it is
possible to read properties of the target.

Seems like there's some sort of weird scope issue going on here.

Thoughts?

[1] https://github.com/delcypher/cmake_add_custom_command_bug
[2] https://github.com/delcypher/cmake_add_custom_command_bug/blob/master/test/CMakeLists.txt

Thanks,
Dan Liew