title: Configuring DUB to use 64-bit compiler
body: <p>How do I configure DUB to compile my application as 64-bit executable? Here's my dub.json:</p>
  
  <pre><code>{
      "name": "dvulkanbase",
      "targetType": "executable",
      "description": "Vulkan boilerplate",
      "authors": ["Myself"],
      "homepage": "http://something",
      "license": "MIT"
  }
  </code></pre>
  
  <p>I tried adding this line to dub.json:</p>
  
  <pre><code>    "dflags-dmd": ["-m64"]
  </code></pre>
  
  <p>but then <code>dub build</code> outputted:</p>
  
  <pre><code>## Warning for package dvulkanbase ##
  
  The following compiler flags have been specified in the package description
  file. They are handled by DUB and direct use in packages is discouraged.
  Alternatively, you can set the DFLAGS environment variable to pass custom flags
  to the compiler, or use one of the suggestions below:
  
  -m64: Use --arch=x86/--arch=x86_64/--arch=x86_mscoff to specify the target architecture
  
  Performing "debug" build using dmd for x86.
  </code></pre>
  
  <p>So I tried replacing the line with:</p>
  
  <pre><code>"dflags-dmd": ["--arch=x86_64"]
  </code></pre>
  
  <p>but got this error:</p>
  
  <pre><code>Error: unrecognized switch '--arch=x86_64'
  </code></pre>
  
  <p>I'm on Windows 10, have DMD 2.074.0 and Visual Studio 2015 and 2017 installed.</p>