add fast-fail feature for pending builds in PR's

appveyor.yml

@@ -15,6 +15,10 @@ environment:
     ARCH: 32
 build_script:
 - ps: |
+    if ($env:APPVEYOR_PULL_REQUEST_NUMBER -and $env:APPVEYOR_BUILD_NUMBER -ne ((Invoke-RestMethod `
+      https://ci.appveyor.com/api/projects/$env:APPVEYOR_ACCOUNT_NAME/$env:APPVEYOR_PROJECT_SLUG/history?recordsNumber=50).builds | `
+      Where-Object pullRequestId -eq $env:APPVEYOR_PULL_REQUEST_NUMBER)[0].buildNumber) { `
+      throw "There are newer queued builds for this pull request, failing early." }
     mkdir build
     cd build
     if ($env:GENERATOR -ne "MSYS Makefiles") {