BLD: Cancel previously queued appveyor builds for a PR

appveyor.yml

@@ -77,6 +77,16 @@ init:
   - "ECHO %NUMPY_VERSION%"
 
 install:
+  # If there is a newer build queued for the same PR, cancel this one.
+  # The AppVeyor 'rollout builds' option is supposed to serve the same
+  # purpose but it is problematic because it tends to cancel builds pushed
+  # directly to master instead of just PR builds (or the converse).
+  # credits: JuliaLang developers.
+  - 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." }
+
   # Get cygwin's git out of our PATH.  See https://github.com/omnia-md/conda-dev-recipes/pull/16/files#diff-180360612c6b8c4ed830919bbb4dd459
   - "del C:\\cygwin\\bin\\git.exe"
   # this installs the appropriate Miniconda (Py2/Py3, 32/64 bit),