CMake: make sure to add RPATH to area-native library

[wwmayer]

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[freecadci]

for feature branch PR_8328. Pipeline 765366634 was triggered at 110b339. All CI branches and pipelines.

[0penBrain]

Thanks for this @wwmayer. I'm checking this PR would pass CI with 20.04 build enabled. If succeeds, I'll commit everything in a single shot.

[0penBrain]

Merged as 681b809

Thanks