cmake: Only enable CXX when running the coverage

Thomas Peralerequested to merge
(removed):fix-cross-compilation-cxx into master

Buildroot builds regularly fails because of libssh package (see http://autobuild.buildroot.org/results/683/683e4eeada3c3291dc09e096dbd9edeed3fb1eec/build-end.log for instance).

The commit 25a67819 introduced that dependency related to code coverage. This patch propose to move that dependency to be only enabled when running that coverage. This fix the issue we run into in buildroot.

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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