[PATCH] Keychain conditions fix-up for Darwin / Python

Hi!

I've made the attached patch to address an issue with the Python bindings on Darwin. The patch itself is attached and details about the problem follow below.

The problem was noticed when I tried to run the following Python code:

python -c 'import svn.client'

In this case Subversion has been configured with the switch "--disable-keychain", I assume
this is triggering the problem.

I am not too deep into C, my best guess is though that a mix of IF conditions was leading into this situation. That's why I unified them for the keychains. With the change applied the the import was working fine again.

The error output I was seeing originally was this:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/python2.7/site-packages/svn/client.py", line 26, in <module>
    from libsvn.client import *
  File "/nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/python2.7/site-packages/libsvn/client.py", line 28, in <module>
    _client = swig_import_helper()
  File "/nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/python2.7/site-packages/libsvn/client.py", line 24, in swig_import_helper
    _mod = imp.load_module('_client', fp, pathname, description)
ImportError: dlopen(/nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/python2.7/site-packages/libsvn/_client.so, 2): Symbol not found: _svn_auth__get_keychain_simple_provider
  Referenced from: /nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/libsvn_subr-1.0.dylib
  Expected in: flat namespace
 in /nix/store/6h83s2gyk8mi7prayji0blcflmrm5rmi-subversion-1.9.4/lib/libsvn_subr-1.0.dylib

Cheers, Johannes