title: How to build object files only once with cmake?
body: <p>I have a CMakeLists.txt file that looks like this:</p>
  
  <pre><code>add_executable(exec1 exec1.c source1.c source2.c source3.c)
  add_executable(exec2 exec2.c source1.c source2.c source3.c)
  add_executable(exec3 exec3.c source1.c source2.c source3.c)
  </code></pre>
  
  <p>The source1.o source2.o source3.o files take a really long time to build, and since they are common to all the executables, I want each of them to be built only once. The current behavior for cmake, however, is to rebuild them for each exec target separately, which is an unnecessary duplication of effort.</p>
  
  <p>Is there a way to tell cmake to build object files only once?</p>