Example 17 crashes on Windows platform

[unknownue]

When run example 17 on Windows, the program crashes and output the following message:

thread 'main' panicked at 'Failed to wait for Fence!: Result(-4)', src\libcore\result.rs:999:5
note: Run with 'RUST_BACKTRACE=1' environment variable to display a backtrace.
error: process didn't exit successfully: 'target\debug\17.exe' (exit code: 101)

Related issue: #10

[unknownue]

Example 17 is an incomplete example, and example 18 is its complete version. On my windows machine, example 17 crashes, but example 18 works well. Currently I failed to find any explicit method to keep example 17 incomplete but not crash on Windows. So if example 17 crashes on your machine, ignore it and just go through to example 18.