title: How to check the OS with automake
body: <p>I have a project that uses automake to create the <code>configure</code> and all related files  (I'm using <code>autoreconf</code> command to make all this stuff). So, I'm trying to set some conditional files to compile when the project is compiling for macOS (OS X), Windows or Linux. But it fails with the following:</p>
  
  <pre><code> $ autoreconf -i ..
  src/Makefile.am:30: error: LINUX does not appear in AM_CONDITIONAL
  autoreconf: automake failed with exit status: 1
  </code></pre>
  
  <p>And the part containing the error in that <code>Makefile.am</code> is the following:</p>
  
  <pre><code>if OSX
      butt_SOURCES += CurrentTrackOSX.h CurrentTrackOSX.m
  endif
  if LINUX
      butt_SOURCES += currentTrack.h currentTrackLinux.cpp
  endif
  if WINDOWS
      butt_SOURCES += currentTrack.h currentTrack.cpp
  endif
  </code></pre>
  
  <p>My question is, how can I check if the OS is Linux? And if it's possible, is there a better way to check the OS in automake?</p>