...

CMakeLists.txt

@@ -82,4 +82,12 @@ if(UNIX)
     if(PK_ENABLE_OS)
         target_link_libraries(${PROJECT_NAME} dl)
     endif()
+
+    # for very old linux systems
+    include(CheckLibraryExists)
+    check_library_exists(rt clock_gettime "" HAVE_LIBRT)
+
+    if(HAVE_LIBRT)
+        target_link_libraries(${PROJECT_NAME} rt)
+    endif()
 endif()