setup.py: fix for Python 2.6

With Python 2.6 on Mac OS X 10.11 I got an error:
$ ./setup.py install
running install
running build_ext
Traceback (most recent call last):
  File "./setup.py", line 195, in <module>
    setup(**kw)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/core.py", line 151, in setup
    dist.run_commands()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/dist.py", line 972, in run_command
    cmd_obj.run()
  File "./setup.py", line 103, in run
    if self._called_from_setup:
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/distutils/cmd.py", line 105, in __getattr__
    raise AttributeError, attr
AttributeError: _called_from_setup

Now _called_from_setup() is called only if it exists.

setup.py

@@ -100,7 +100,12 @@ def run(self):
  if self.old_and_unmanageable or self.single_version_externally_managed:
  return install_orig.install.run(self)
 
- if not self._called_from_setup(inspect.currentframe()):
+ try:
+ have_called_from_setup = self._called_from_setup is not None
+ except AttributeError:
+ have_called_from_setup = False
+
+ if not have_called_from_setup or not self._called_from_setup(inspect.currentframe()):
  # Run in backward-compatibility mode to support bdist_* commands.
  install_orig.install.run(self)
  else: