CMake: fix absolute include install dir

[nim65s]

Hi,

Users may want to provide absolute paths in CMAKE_INSTALL_INCLUDEDIR. When this is the case, we should not prefix it in any way.

Without this, building https://github.com/gazebosim/gz-transport v15 on nixos currently fail with:

CMake Error in test/integration/CMakeLists.txt:
  Imported target "zenohcxx_zenohc" includes non-existent path

    "/nix/store/8vadygf30zr2qlk8cqnnsdjc2h8a72cm-zenoh-cpp-1.4.0//nix/store/8vadygf30zr2qlk8cqnnsdjc2h8a72cm-zenoh-cpp-1.4.0/include"

edit: fix relocatable setup

[github-actions]

PR missing one of the required labels: {'api-sync', 'new feature', 'breaking-change', 'dependencies', 'enhancement', 'ci', 'documentation', 'internal', 'bug'}

[github-actions]

PR missing one of the required labels: {'new feature', 'bug', 'internal', 'enhancement', 'dependencies', 'documentation', 'breaking-change', 'ci', 'api-sync'}