fix cygwin64 gc

http://cygwin.com/ml/cygwin-apps/2013-04/msg00281.html

`make check` freezed on some test although.

    test sigalrm4 (interrupting syscall), expects 14 ==> ERROR: GOT #<<unhandled-signal-error> "unhandled signal 2 (SIGINT)">
    test sigalrm5 (interrupting syscall - restart), expects (a) ==> ERROR: GOT #<<unhandled-signal-error> "unhandled signal 2 (SIGINT)">