Errors with catkin_make and beginner_tutorials

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I'm new to ROS. I followed the tutorials and run cd ~/catkin_ws and catkin_make, but the output has some warnings and errors, following is the output:

Base path: /home/xu/catkin_ws
Source space: /home/xu/catkin_ws/src
Build space: /home/xu/catkin_ws/build
Devel space: /home/xu/catkin_ws/devel
Install space: /home/xu/catkin_ws/install
####
#### Running command: "make cmake_check_build_system" in "/home/xu/catkin_ws/build"
####
-- Using CATKIN_DEVEL_PREFIX: /home/xu/catkin_ws/devel
-- Using CMAKE_PREFIX_PATH: /opt/ros/indigo
-- This workspace overlays: /opt/ros/indigo
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using Debian Python package layout
-- Using empy: /usr/bin/empy
-- Using CATKIN_ENABLE_TESTING: ON
-- Call enable_testing()
-- Using CATKIN_TEST_RESULTS_DIR: /home/xu/catkin_ws/build/test_results
-- Found gtest sources under '/usr/src/gtest': gtests will be built
-- Using Python nosetests: /usr/bin/nosetests-2.7
-- catkin 0.6.14
-- BUILD_SHARED_LIBS is on
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- ~~  traversing 1 packages in topological order:
-- ~~  - beginner_tutorials
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- +++ processing catkin package: 'beginner_tutorials'
-- ==> add_subdirectory(beginner_tutorials)
**CMake Warning (dev) in beginner_tutorials/CMakeLists.txt:
  Syntax Warning in cmake code at****strong text**

    /home/xu/catkin_ws/src/beginner_tutorials/CMakeLists.txt:11:14

  Argument not separated from preceding token by whitespace.
This warning is for project developers.  Use -Wno-dev to suppress it.

**CMake Warning** (dev) in beginner_tutorials/CMakeLists.txt:
  Syntax Warning in cmake code at

    /home/xu/catkin_ws/src/beginner_tutorials/CMakeLists.txt:11:29

  Argument not separated from preceding token by whitespace.
This warning is for project developers.  Use -Wno-dev to suppress it.

**CMake Error** at /opt/ros/indigo/share/catkin/cmake/catkinConfig.cmake:75 (find_package):
  Could not find a package configuration file provided by "<span" with any of
  the following names:

    <spanConfig.cmake
    <span-config.cmake

  Add the installation prefix of "<span" to CMAKE_PREFIX_PATH or set
  "<span_DIR" to a directory containing one of the above files.  If "<span"
  provides a separate development package or SDK, be sure it has been
  installed.
Call Stack (most recent call first):
  beginner_tutorials/CMakeLists.txt:7 (find_package)


-- Configuring incomplete, errors occurred!
See also "/home/xu/catkin_ws/build/CMakeFiles/CMakeOutput.log".
See also "/home/xu/catkin_ws/build/CMakeFiles/CMakeError.log".
make: *** [cmake_check_build_system] Error 1
**Invoking "make cmake_check_build_system" failed**