Fix symlink install

[tpoignonec]

No description provided.

[tpoignonec]

Remaining issue: how to detect a symlink install flag?

If possible (e.g., AMENT_SYMLINK_INSTALL or such), the following should work:

if(DEFINED ENV{AMENT_SYMLINK_INSTALL})
  message(STATUS "Symlink install is enabled.")
  # Symlink-install-specific logic here
else()
  message(STATUS "Symlink install is NOT enabled.")
  # Normal install logic here
endif()