Improve CMake

[Rombur]

This PR allows to build the mfmg as a shared library even when CUDA is enabled.

@aprokop can you try this PR on your machine

[aprokop]

Sure, once I get dealii installed :)

[codecov-io]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   87.25%   87.25%           
=======================================
  Files          27       27           
  Lines        1765     1765           
=======================================
  Hits         1540     1540           
  Misses        225      225

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[Rombur]

@aprokop I know this does not pass the tests but can you still try it. I would be interested to see if the problem is reproducible outside docker.

[Rombur]

I am giving up on this for now. While removing the stubs fixes the problem in docker it seems that stubs are not the real reason of the problem.