Add 'System' to the list of libc equivalents

This is apparently needed on some OSX systems. Closes #188.

Author: bmerry

fakeredis.py

@@ -88,7 +88,7 @@ def iteritems(d):
 
 DATABASES = {}
 
-_libc_library = find_library('c') or find_library('msvcrt')
+_libc_library = find_library('c') or find_library('msvcrt') or find_library('System')
 
 if not _libc_library:
     raise ImportError('fakeredis: unable to find libc or equivalent')

test_fakeredis.py

@@ -3880,7 +3880,7 @@ def test_can_pass_through_extra_args(self):
 class TestImportation(unittest.TestCase):
     def test_searches_for_c_stdlib_and_raises_if_missing(self):
         """
-        Verifies that fakeredis checks for both libc and msvcrt when
+        Verifies that fakeredis checks for multiple C library implementations
         looking for a strtod implementation and that it fails fast when neither
         is found.
         """
@@ -3898,7 +3898,7 @@ def test_searches_for_c_stdlib_and_raises_if_missing(self):
             with self.assertRaises(ImportError):
                 reload(fakeredis)
 
-            self.assertEqual(set(['c', 'msvcrt']), searched_libraries)
+            self.assertEqual(set(['c', 'msvcrt', 'System']), searched_libraries)
         finally:
             ctypes.util.find_library = old_find_library