anchor.diff:  The Ninja log
   ~~~~~~~~~~~~~
   
  +[[ref_log]]
  +
   For each built file, Ninja keeps a log of the command used to build
   it.  Using this log Ninja can know when an existing output was built
   with a different command line than the build files specify (i.e., the
  
positive.code: The log file is kept in the build root in a file called `.ninja_log`.
  If you provide a variable named `builddir` in the outermost scope,
  `.ninja_log` will be kept in that directory instead.
  
  
  Ninja file reference
  
negative.code: considered part of its parent's scope; if it is indented less than the
  previous one, it closes the previous scope.
  
  Top-level variables
  ~~~~~~~~~~~~~~~~~~~
  
  Two variables are significant when declared in the outermost file scope.
  
  `builddir`:: a directory for some Ninja output files.  See <<ref_log,the
    discussion of the build log>>.  (You can also store other build output
    in this directory.)
  
  `ninja_required_version`:: the minimum verison of Ninja required to process
    the build correctly.  See <<ref_versioning,the discussion of versioning>>.
  
  
  Rule variables
  ~~~~~~~~~~~~~~
  [[ref_rule]]
  