Exclude sources from unity builds

[steinmig]

The library has some multiple variable names and macro names, leading to One Definition Rule problems with unity builds.

This cmake update excludes all sources from unity builds, letting them be compiled "normally"

[atztogo]

SKIP_UNITY_BUILD_INCLUSION is supported by cmake 3.16 or later. I'm not sure which cmake versiont we should require to users. @steinmig, is it possible to skill this section if cmake version is not support it?

[steinmig]

Sorry, I just now noticed that you actually support all the way to 2.8.

I don't have a CMake 2 laying around, but I think it should support the current solution

[codecov-commenter]

Codecov Report

Base: 85.12% // Head: 85.12% // No change to project coverage 👍

Coverage data is based on head (dda25d3) compared to base (de222b7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #191   +/-   ##
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  Coverage    85.12%   85.12%           
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  Files           16       16           
  Lines         1318     1318           
========================================
  Hits          1122     1122           
  Misses         196      196           

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[atztogo]

Merged. @steinmig, thank you very much!

[LecrisUT]

@steinmig Would you be so kind to double-check if the recent v2.1.0-rc1 still needs this?

[steinmig]

@LecrisUT Yes this is still needed, since there are still variables / functions with identical names.

[LecrisUT]

Could you walk me through on how to generate an error without this set? As you can see this is currently disabled, and we might not have the appropriate tests to detect an error from this.

[steinmig]

Execute cmake with the option for a unity build like:

cmake -DCMAKE_UNITY_BUILD=true ..

Basically this fix simply disables this option for all the files of this project, which can be useful if this is a dependency for a project that does use this option.
The benefit of this option is a shorter compile time, which we exploit for CI purposes.

(the option is usually no problem for C++ due to namespaces, but of course this is not a solution here. There are actually only a few duplicates, but I did not want to change the names, because one is function and I suspect this would be API breaking, although I didn't have a closer look at that)