title: Unable to get the wrap-file option in meson build system working
body: <p>I was trying to cross compile glib for arm. It has a dependency on libffi, but found that a meson build for libffi was last updated 2 yrs back and there is a problem with what is available.</p>
  
  <p>Therefore i had to define a wrap file to take the latest source of libffi.</p>
  
  <p>My first idea was to use a wrap file as below -</p>
  
  <pre><code>    [wrap-git]
      directory=libffi
      url=https://github.com/libffi/libffi
      revision=head
  </code></pre>
  
  <p>p.s its the head revision that has the right source code for libffi. But it needed a meson revision and doesn't seem to take the head.</p>
  
  <p>Therefore i went for the [wrap-file] option,</p>
  
  <pre><code>    [wrap-file]
      directory = libffi-3.3
      source_url = https://github.com/libffi/libffi/releases/download/v3.3/libffi-3.3.tar.gz
      source_filename = libffi-3.3.tar.gz
      source_hash = 72fba7922703ddfa7a028d513ac15a85c8d54c8d67f55fa5a4802885dc652056
  </code></pre>
  
  <p>But still i get this error "Subproject exists but has no meson.build file" when doing a meson build.</p>
  
  <p>This is the original wrap file that came alongwith glib.</p>
  
  <pre><code>    [wrap-git]
      directory=libffi
      url=https://github.com/centricular/libffi.git
      revision=meson
  </code></pre>
  
  <p>what is the easiest way to get a wrap file working.</p>