io.c: max_file_descriptor

* io.c (max_file_descriptor): rb_atomic_t for ATOMIC_CAS().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39776 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
mingw build fix

* configure.in: struct timeval is defined in winsock2.h on mingw.

* include/ruby/missing.h: include time.h for time_t, and sys/time.h
  for timeval and timespec.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39777 b2dd03c8-39d4-4d8f-98ff-823fe69b080e