adjust dependency check for gnureadline on OS X

- always depend when making a bdist (for consistency)
- otherwise only depend if missing (e.g. pip install -e)

setup.py

@@ -279,7 +279,8 @@ for deps in extras_require.values():
 extras_require['all'] = everything
 install_requires = []
 if sys.platform == 'darwin':
-    install_requires.append('gnureadline')
+    if any(arg.startswith('bdist') for arg in sys.argv) or not setupext.check_for_readline():
+        install_requires.append('gnureadline')
 elif sys.platform.startswith('win'):
     # Pyreadline has unicode and Python 3 fixes in 2.0
     install_requires.append('pyreadline>=2.0')