title: How to specify requirements conditionally in `conanfile.txt`?
body: <p>I have to deal with a cross-platform application which needs an additional 3rd party library on Windows but not on Linux (and that library doesn't support Linux anyway). I have packed the library into a Conan package, supporting only `os=Windows complier="Visual Studio". Now if I put this library as</p>
  
  <pre><code>[requires]
  Library/1.2.3@foo/bar
  </code></pre>
  
  <p>in my <code>conanfile.txt</code>, <code>conan install</code> will logically fail with the Error <code>Missing prebuilt package</code> on Linux.</p>
  
  <p>So is there a way to specify required packages conditionally in <code>conanfile.txt</code>? Something like <code>Library/1.2.3@foo/bar [os="Windows"]</code>. I read through the Conan docs but found nothing.</p>
  
  <p>Or to tell <code>conan install</code> ignore the error?</p>
  
  <p>Or my only luck is to use two different <code>conanfile.txt</code> on two platforms?</p>
  
  <p>I can not use a <code>conanfile.py</code>, since the build process is not managed by Conan, only the dependencies.</p>