[CMake] race condition with target_objects

[Juan E. Sanchez]

Hello Brad,

Thanks for finding the issue.  I had simply replaced the shared library 
I was using with the name of the TARGET_OBJECTS generator.

The shared objects are great.  It helps me bypass Apple security issues 
when a system interpreter loads a shared library, which loads another 
shared library.

Regards,

Juan

On 6/26/18 1:34 PM, Brad King wrote:
> 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
>