Do not install dirent.h on Windows to avoid conflicts

[traversaro]

dirent.h is not used in public headers of Simbody, so we can avoid to install it as it can create conflicts with other installation of dirent.h from https://github.com/tronkko/dirent .

See for the downstream issue: conda-forge/simbody-feedstock#39 .

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This change is 

[traversaro]

The Travis CI failures seems not to be related with the content of this PR.

[chrisdembia]

Does this PR potentially remove other headers that Simbody depends on?

In general, Simbody, on Windows, is installed into its own directory, not in system directories. I don't feel too strongly one way or the other, so I'll defer to @sherm1, but perhaps having the patch for conda is the right move. We could also potentially add a CMake option to control this behavior.

Another test would be to build a project that is downstream of Simbody, using the Simbody installation.

[traversaro]

Does this PR potentially remove other headers that Simbody depends on?

No, the only include in any platform dir is dirent.h : https://github.com/simbody/simbody/tree/master/Platform/Windows/include and dirent is not used in any

but perhaps having the patch for conda is the right move

Sure, I opened the PR for visibility and it could be useful for other package managers users on Windows (vcpkg, conan, spack). However if you prefer that we just have the patch on Conda, this is not a problem.

Another test would be to build a project that is downstream of Simbody, using the Simbody installation.

I may be wrong, but couldn't just inspect downstream projects for any use of dirent.h via code search?