compat.h: Add MSG_CMSG_CLOEXEC for older glibc.

This patch fixes this build error with glibc before 2.7.

  CC    syscalls/recvfrom.o
syscalls/recvfrom.c:28: error: ‘MSG_CMSG_CLOEXEC’ undeclared here (not in a function)
make: *** [syscalls/recvfrom.o] Error 1

Signed-off-by: Vinson Lee <vlee@twitter.com>