title: Error building GDAL: PROJ_INCLUDE should be defined. PROJ >= 6 is a required dependency
body: <p>I am trying to get GDAL 3.0.1 to work. Generating the Visual Studio 2017 project was a piece of cake, all I needed was to run <code>generate_vcxproj.bat</code>. But now I'm getting an error on this line of <code>makefile.vc</code>:</p>
  
  <pre><code>!IF !DEFINED(PROJ_INCLUDE)
  !ERROR PROJ_INCLUDE should be defined. PROJ &gt;= 6 is a required dependency
  !ENDIF
  </code></pre>
  
  <p>What's that all about? If I am meant to define <code>PROJ_INCLUDE</code>, what value should it have? Is that even preprocessor define?</p>