What for catkin needs to link to librt (realtime extension)?

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(Setting aside my knowledge about build system is limited,)

There's a macro in catkin: catkin/cmake/tools/rt.cmake, inside of which it tries to find librt I think. Does catkin need realtime extension for:

• building catkin itself?

OR

• building whatever packages that require rt?

or both?

I'm wondering, because while trying to build catkin workspace from source (following this doc) on non-Ubuntu platform (QNX), I get this error:

# cd catkinws && mkdir build && cd build && cmake ../src
:
-- Found gtest sources under '/home/rosuer/catkin_ws/src/gtest': gtests will be built
CMake Error at catkin/cmake/assert.cmake:17 (message):

  Assertion failed: check for file existence, but filename
  (RT_LIBRARY-NOTFOUND) unset.  Message: RT Library

Call Stack (most recent call first):
  catkin/cmake/tools/rt.cmake:42 (assert_file_exists)
  catkin/cmake/all.cmake:134 (include)
  CMakeLists.txt:12 (include)

-- Configuring incomplete, errors occurred!

While remaining not so sure, I modified a line of rt.cmake as follows that bypass the error above on QNX. And no build-time error occurs on Ubuntu (Quantal) too (I don't know if there's any runtime misbehavior):

x if(NOT (APPLE OR WIN32 OR MINGW OR ANDROID))
o if(NOT (APPLE OR WIN32 OR MINGW OR ANDROID OR UNIX))

Although I'm not sure if UNIX symbol is recognized by cmake on Ubuntu/Linux, succeeding this even on Ubuntu made me wonder why linking to rt is needed in catkin level.

Thank you!