title: CMAKE Parallel Build issue
body: <p>I have external Project which is make based, I am able to compile link successfully. ( I have modified make infra of third party project to break large make into multiple smaller makes so that parallelly different small make can run, but it is orthogonal to this question) </p>
  
  <p>Third party cmake  is as below </p>
  
  <pre><code>External_Project_Add( ...
               ....
               #NO BUILD_COMMAND) 
  </code></pre>
  
  <p>But Compilation is too slow, and I can see that max number of compilation doesn't exceed 8 threads and CPU utilization is ~20%.  (we are using distcc as well). 
  If I change as below  </p>
  
  <pre><code>External_Project_Add( ...
               ....
           BUILD_COMMAND ${CMAKE_COMMAND} --build &lt;dir&gt; --parallel --)
  </code></pre>
  
  <p>Compilation is super-fast by atleast 3 time. However occasionally system run out of PIDs. </p>
  
  <p>Is there way to control max number of PIDs/process    available to CMAKE ?</p>
  
  <p>I had thought about using -j option is BUILD_COMMAND but it is artificially going to reduce the compilation compute available to my system. </p>
  
  <p>I have tried changing $(MAKE) in my third party make to have -jn, but then it shows jobserver domain warning. It is not good idea to change third party make as well. So I dropped this idea. </p>
  
  <p>Any help is appreciated !</p>