Fix python3's compileall command for build.py file

Because as of Python 3.5, the .pyo filename extension is no longer used, see:
  - `PEP 488 -- Elimination of PYO files` (https://www.python.org/dev/peps/pep-0488/)
  - `PEP 3147 -- PYC Repository Directories` as to why a separate directory is used (https://www.python.org/dev/peps/pep-3147/)

Also moves comment to the right place

Author: opacam

pythonforandroid/bootstraps/common/build/build.py

@@ -253,11 +253,15 @@ def compile_dir(dfn, optimize_python=True):
     if get_bootstrap_name() != "sdl2":
         # HISTORICALLY DISABLED for other than sdl2. NEEDS REVIEW! -JonasT
         return
-    # -OO = strip docstrings
     if PYTHON is None:
         return
-    args = [PYTHON, '-m', 'compileall', '-f', dfn]
+
+    if int(PYTHON_VERSION[0]) >= 3:
+        args = [PYTHON, '-m', 'compileall', '-b', '-f', dfn]
+    else:
+        args = [PYTHON, '-m', 'compileall', '-f', dfn]
     if optimize_python:
+        # -OO = strip docstrings
         args.insert(1, '-OO')
     subprocess.call(args)