title: GPRBuild does not find gprconfig when called from the Windows Subsystem for Linux
body: <p>I have installed GNAT GPL 2017 on my Windows 10 system. Now I want to call the compiler from within a linux shell running on the Windows Subsystem for Linux. By adding the <code>bin</code> directory of my GNAT installation to the PATH, I got the following to work:</p>
  
  <pre><code>$ gnatmake.exe -P yaml.gpr
  </code></pre>
  
  <p>However, using GPRBuild does not work:</p>
  
  <pre><code>$ gprbuild.exe -P yaml.gpr
  gprbuild: could not locate gprconfig for auto-configuration
  </code></pre>
  
  <p>I tried to call gprconfig from the command line and got this:</p>
  
  <pre><code>$ gprconfig.exe
  No compilers found for target i686-pc-mingw32
  </code></pre>
  
  <p>It seems like gprbuild and gprconfig have problems with locating the GNAT binaries, while gnatmake is perfectly able to. How do I get gprbuild to work? Is there some environment variable I need to set?</p>