title: CMake add -ldl at end of link stage of add_library
body: <p>I wrote/maintain a small unit test library on github <a href="https://github.com/acgreek/ExtremeCUnit" rel="nofollow">https://github.com/acgreek/ExtremeCUnit</a> built using cmake. You can checkout and run some  tests via 'cmake test' after checking out. It was working great on Cygwin and Ubuntu (my only home systems). I recent upgrade to ubuntu 13.10 and the library stopped linking with the test binary because the ExtremeUnitC library now needs to be linked with -ldl at the link stage (via add_library) and additionally the -ldl needs to be add to the end of the link line (some change to gcc it seems). In prior version of Ubuntu, the add_library target didn't need the -ldl  until the test object  was linked with the ExtremeUnitC library </p>
  
  <p>Basically what I need is for the results of make VERBOSE=10, when it get to the following stage</p>
  
  <pre><code>Linking C shared library libExtremeCUnit.so
  /usr/bin/cmake -E cmake_link_script CMakeFiles/ExtremeCUnit.dir/link.txt --verbose=10
  /usr/bin/gcc  -fPIC -Wall -Wextra -ggdb3 -fPIC  -ldl   -shared -Wl,-    soname,libExtremeCUnit.so -o libExtremeCUnit.so CMakeFiles/ExtremeCUnit.dir/main.c.o CMakeFiles/ExtremeCUnit.dir/runner.c.o CMakeFiles/ExtremeCUnit.dir/util.c.o CMakeFiles/ExtremeCUnit.dir/findtest_names.c.o CMakeFiles/ExtremeCUnit.dir/assert_support.c.o 
  </code></pre>
  
  <p>I need it to be, </p>
  
  <pre><code>Linking C shared library libExtremeCUnit.so
  /usr/bin/cmake -E cmake_link_script CMakeFiles/ExtremeCUnit.dir/link.txt --verbose=10
  /usr/bin/gcc  -fPIC -Wall -Wextra -ggdb3 -fPIC  -ldl   -shared -Wl,-    soname,libExtremeCUnit.so -o libExtremeCUnit.so CMakeFiles/ExtremeCUnit.dir/main.c.o CMakeFiles/ExtremeCUnit.dir/runner.c.o CMakeFiles/ExtremeCUnit.dir/util.c.o CMakeFiles/ExtremeCUnit.dir/findtest_names.c.o CMakeFiles/ExtremeCUnit.dir/assert_support.c.o -ldl
  </code></pre>
  
  <p>how should I edit my CMakeList.txt to that in a clean/portable way?</p>
  
  <p>You can also send me a pull request so you can get the credit of fixing it.</p>