Question

How to write CMakeLists.txt to conditionally link to the system-wide librt library only when on Linux environment?

Answer 1

cmake has several predefined variables useful for environment detection (WIN32, UNIX, APPLE, CYGWIN). Here is the full list: http://www.cmake.org/cmake/help/cmake-2-8-docs.html#section_VariablesThatDescribetheSystem

So you can write something like

if(UNIX AND NOT APPLE)
    target_link_libraries(target_name rt)
endif()