[CMake] race condition with target_objects

[Brad King]

On 06/26/2018 01:32 PM, Juan E. Sanchez wrote:
> The likely problem is that the symdiff_objects and the symdiff_tcl are 
> in side-by-side directories.  They are both added using ADD_SUBDIRECTORY 
> one level up.  If I put symdiff_python before symdiff_tcl, then that 
> target will fail.  Please fix your Makefile generator.

Target dependencies are agnostic to the directories.  The ordering at
most matters for serial builds of independent targets.

While trying to reproduce this in a simple example I found the problem:

```
TARGET_LINK_LIBRARIES (symdiff_tcl $<TARGET_OBJECTS:symdiff_objects> ${TCL_ARCHIVE})
```

That is not the way to reference an object library.

The documentation:

  https://cmake.org/cmake/help/v3.12/manual/cmake-buildsystem.7.html#object-libraries

shows that `$<TARGET_OBJECTS:...>` belongs in the list of *sources*,
not linked libraries.  You should be using:

```
add_library(symdiff_tcl SHARED ... $<TARGET_OBJECTS:symdiff_objects>)
```

-Brad