[Openmp-dev] Cmake changes

[C Bergström]

Rant: I'll send more details and a patch as soon as I have one, but I
must say - every time someone fiddles with the cmake in this project
it gets broken.
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I don't know if anyone else is hitting this, but I'm getting random
build failures and some problems with building it statically.

Hopefully this last batch of changes will soon lead to a long period
of stability.