Avoid to use the old GenerateExportHeader included with VTK 6

[traversaro]

vtk 6.3 installs a GenerateExportHeader CMake module that shadows the official CMake module if find_package(VTK) is invoked.

To ensure that the build works even if find_package(VTK) is invoked and VTK 6 is found, we manually include exactly the
GenerateExportHeader that is shipped with CMake, extracting the CMake install prefix from the CMAKE_COMMAND variable

The issue has been solved in vtk >= 7.1, see https://gitlab.kitware.com/vtk/vtk/-/merge_requests/1593 , so this workaround can be removed once we will not support anymore platforms that ship with vtk 6.3 as main vtk version (Ubuntu 18.04).

Fix #714 that was happening on a project that included iDynTree via add_subdirectory and indirectly was calling find_package(VTK) with VTK 6, via find_package(PCL).

[traversaro]

VTK users that still use Ubuntu 18.04 may need to be aware of this: @pattacini @xEnVrE @vvasco .

[traversaro]

The CMAKE_COMMAND workaround is not working on macOS, apparently.

[traversaro]

The CMAKE_COMMAND workaround is not working on macOS, apparently.

Thanks to @lrapetti, it seems that on macOS/Homebrew CMake installs its modules in <prefix>/share/cmake while on linux it installs them in <prefix>/share/cmake-3.16 . As this workaround is something that mostly is related to old linux distributions, I think it is sufficient to just check if the GenerateExportHeader.cmake file exists, and if it does not exist we can just fall back on just include(GenerateExportHeader) .