Issue #12326: refactor usage of sys.platform

 * Use str.startswith(tuple): I didn't know this Python feature, Python rocks!
 * Replace sometimes sys.platform.startswith('linux') with
   sys.platform == 'linux'
 * sys.platform doesn't contain the major version on Cygwin on Mac OS X
   (it's just 'cygwin' and 'darwin')

command/build_ext.py

@@ -240,8 +240,7 @@ class build_ext(Command):
         # for extensions under Linux or Solaris with a shared Python library,
         # Python's library directory must be appended to library_dirs
         sysconfig.get_config_var('Py_ENABLE_SHARED')
-        if ((sys.platform.startswith('linux') or sys.platform.startswith('gnu')
-             or sys.platform.startswith('sunos'))
+        if (sys.platform.startswith(('linux', 'gnu', 'sunos'))
             and sysconfig.get_config_var('Py_ENABLE_SHARED')):
             if sys.executable.startswith(os.path.join(sys.exec_prefix, "bin")):
                 # building third party extensions