Document how to make tests depend on optional dune modules

CMAKE_GUARD allows to do that, but the syntax was not clear. This merge request adds a short description.

Please let me check why I remeber that once HAVE_DUNE_*worked.

Ok, my mistake, it was always dune-*_FOUND.

@christi This naming scheme is perfectly fine within CMake. HAVE_* is only the syntax for pre-processor macros.

my fault! I misunderstood Olis questions and thought he is talking about a test in an application and not the application itself.

Added 10 commits:

• 3ebc3d09...a6d255fd - 8 commits from branch master
• 6dfac46b - Fix a few typos
• 6605c62d - Document how to make CMAKE_GUARD check for Dune modules

Enabled an automatic merge when the build for 6605c62d succeeds

Canceled the automatic merge

mentioned in commit 16ac25a9

Status changed to merged

Mentioned in commit 16ac25a9