Bug 1100851 - Tweak DMD to account for the fact that $DMD can now be undefined. r=glandium.

Now that defining $DMD is no longer necessary to run DMD, this patch does the
following.
- Removes all the places where we set DMD=1 (test harnesses, etc.)
- Still handles DMD=1, for backwards compatibility.
- Prints "$DMD is undefined" at DMD start-up if appropriate.
- Writes a |null| value for |dmdEnvVar| in the JSON if $DMD is undefined. Bumps
  the DMD output version number accordingly.
- Changes a bunch of the test files accordingly, including changing the mode of
  script-ignore-alloc-fns.json in order to test a case where $DMD is undefined.

build/automationutils.py

@@ -389,7 +389,6 @@ def environment(xrePath, env=None, crashreporter=True, debugger=False, dmdPath=N
     env[envVar] = os.path.pathsep.join([path for path in envValue if path])
 
   if dmdPath and dmdLibrary and preloadEnvVar:
-    env['DMD'] = '1'
     env[preloadEnvVar] = os.path.join(dmdPath, dmdLibrary)
 
   # crashreporter