CMake: dont concatenate absolute GNUInstallDirs

[nim65s]

Hi,

Building mujoco in nixpkgs, I've just seen a bunch of red flags like [ 88%] Linking CXX shared library ../../nix/store/m96gkn3ack9i93xi0dqzm71g9baz3p9i-mujoco-3.13.0/lib/libsdf_plugin.so, where /nix/store is an absolute path.

In cmake, the GNUInstallDirs CMAKE_INSTALL_*DIR variables can be provided by the end user (or package manager), and therefore they can be absolute.

When that is the case, they should not be concatenated after any other path.

This pull request replace ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_*DIR}" uses by ${CMAKE_INSTALL_FULL_*DIR}, also provided by GNUInstallDirs, which is able to handle correctly the absolute case.

(A nice side-effect of this change is to also handle package relocations: when the user decide to move the package away from build-time CMAKE_INSTALL_PREFIX after installation, as this is for example always the case with the conda package manager)

In a similar way, "${CMAKE_INSTALL_*DIR}" are replaced by a a default bin / lib when absolute and concatenated to ${CMAKE_BINARY_DIR}.