Make build scripts Scons 3.0/Python3 compatible

- Cherry picked #10662 and fixed merge conflicts.
- Manualy merged the change from #11904.
- Did not merge #12236 since I'm not sure whether the issue
  affects Godot 2.1 and I don't have VS2013 to test.
- Did not merge #11843 since it doesn't seem relevant (the
  code is only needed for creating DONORS.md, etc.).
- Did not merge #10727 and #11752 since they seem to be
  already included in #11742.
- The Windows and Linux builds have been tested with Scons 3.0
  using Python 3.
- OSX and iOS should hopefully work but are not tested since
  I don't have a Mac.
- Builds using SCons 2.5 and Python 2 should not be impacted.

platform/javascript/detect.py

@@ -12,11 +12,7 @@ def get_name():
 
 
 def can_build():
-
-    import os
-    if (not os.environ.has_key("EMSCRIPTEN_ROOT")):
-        return False
-    return True
+    return ("EMSCRIPTEN_ROOT" in os.environ)
 
 
 def get_opts():