Always depend on gnureadline if setupext.check_for_readline() fails

The inconsistency (asymmetry) of unconditionally adding gnureadline as a
dependency of binary distributions can cause issues like reported at
https://github.com/paylogic/pip-accel/issues/34

setup.py

@@ -289,7 +289,7 b' install_requires = []'
 
 # add readline
 if sys.platform == 'darwin':
-    if any(arg.startswith('bdist') for arg in sys.argv) or not setupext.check_for_readline():
+    if not setupext.check_for_readline():
         install_requires.append('gnureadline')
 elif sys.platform.startswith('win'):
     extras_require['terminal'].append('pyreadline>=2.0')