MujocoDependencies: Add CMake options to use system versions of dependencies

[traversaro]

This PR adds options for using the system versions of MuJoCo dependencies. This options are OFF by default, but they can be useful for anyone packaging MuJoCo as part of a software distribution.

For each MuJoCo dependencies <dep>, an advanced option (by default OFF) called MUJOCO_USE_SYSTEM_<dep> is created.

To ensure that the MUJOCO_USE_SYSTEM_<dep> option work as intended, the following additional changes were done:

• Use tinyxml2::tinyxml2 target to link tinyxml2 : the tinyxml2::tinyxml2 target works well both when tinyxml2 is found via find_package or via add_subdirectory/FetchContent, as opposed to the tinyxml2 target that only works if tinyxml2 is added with add_subdirectory/FetchContent.
• Use GTest::gtest, GTest::gtest_main and GTest::gmock imported targets instead of gtest, gtest_main and gmock. As n the case of tinyxml2, the target gtest, gtest_main and gmock ar defined only when gtest is included via add_subdirectory.
• Instead, for tinyobjloader and qhull the targets available when the libraries is found via find_package is different from the targets available if the library is included via add_subdirectory, so in this case appropriate alias targets are introduced.
• For CCD, it is necessary to ensure that CCD_STATIC_DEFINE is defined only if the consumed ccd is static, not it if is dynamic (otherwise this will result in Windows linking errors such as https://github.com/conda-forge/libccd-feedstock/issues/8)

[traversaro]

This changes are tested extensively in https://github.com/traversaro/mujoco-conda-forge-ci/pull/2, a CI job in which all dependencies are installed via conda-forge, and not managed by CMake's FetchContent.

[tmplt]

This patch would be useful in NixOS where packages are built in isolated environments (offline with explicit dependencies). The current approach depends on big patches and some messy preConfigure scripts.