replace platform conditions with os_posix and os_bsd where applicable

third_party/libevent/libevent.gyp

 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'variables': {
     'use_system_libevent%': 0,
   },
   'conditions': [
     ['use_system_libevent==0', {
@@ skipping 36 matching lines @@
                 ],
               },
             }],
             [ 'OS == "android"', {
               # On android, epoll_create(), epoll_ctl(), epoll_wait() and
               # clock_gettime() are all in libc.so, so no need to add
               # epoll_sub.c and link librt.
               'sources': [ 'epoll.c' ],
               'include_dirs': [ 'android' ],
             }],
-            [ 'OS == "mac" or OS == "freebsd" or OS == "openbsd"', {
+            [ 'OS == "mac" or os_bsd==1', {
               'sources': [ 'kqueue.c' ],
               'include_dirs': [ 'mac' ]
             }],
             [ 'OS == "solaris"', {
               'sources': [ 'devpoll.c', 'evport.c' ],
               'include_dirs': [ 'solaris' ]
             }],
           ],
         },
       ],
@@ skipping 11 matching lines @@
           'link_settings': {
             'libraries': [
               '-levent',
             ],
           },
         }
       ],
     }],
   ],
 }