skip xamarin-dependent steps in appveyor PR builds

[nathan-schubkegel]

Experimenting based on the discussion started at #1044 (comment)

[nathan-schubkegel]

This is the workaround suggested by Oystein Bjorke at http://help.appveyor.com/discussions/questions/1728-register-xamarin-license-for-pull-requests using windows command line "IF DEFINED" to skip build steps when it's obvious they're going to fail.

IF DEFINED xamarin_password appveyor RegisterXamarinLicense ...

The theory is based on http://www.appveyor.com/docs/lang/xamarin where it says

Secure variables are not decrypted during pull request (GitHub only) 
builds of public projects. This is made intentionally to avoid leaking your 
credentials by submitting PR with malicious build script displaying 
environment variables.

[codecov-io]

Current coverage is 73.05%

Merging #1061 into master will not affect coverage as of 6d8b35e

@@            master   #1061   diff @@
======================================
  Files          263     263       
  Stmts        14897   14897       
  Branches      1623    1623       
  Methods          0       0       
======================================
  Hit          10883   10883       
  Partial        418     418       
  Missed        3596    3596       

Review entire Coverage Diff as of 6d8b35e

---

Uncovered Suggestions

1. +0.09% via ...nternal/AspHelper.cs#206...218
2. +0.09% via ...nternal/AspHelper.cs#188...200
3. +0.09% via ...nternal/AspHelper.cs#170...182
4. See 7 more...

Powered by Codecov. Updated on successful CI builds.

[304NotModified]

thanks! But it seems that it's now never build on Xamarin., See https://ci.appveyor.com/project/nlog/nlog/build/4.0.1278

I expect:

NLog.Xamarin.Android -> C:\projects\nlog\build\bin\Debug\Xamarin.Android\NLog.dll

[nathan-schubkegel]

Yeah. A private CI build (meaning "Wherever the AppVeyor builds work fully for you before these changes") will still run the xamarin stuff because %xamarin_password% expands correctly there. So for git pull request workflow, maybe the idea is at some point, someone does a private CI build to check the Xamarin stuff before accepting a pull request (bleh, more work). Or we just let it go and retroactively fix stuff on 'master' after the fact (ew).

My most recent commit attempted to produce a warning that would make it obvious in the GitHub web display when xamarin steps are skipped (but it doesn't look like it made a noticeable difference like I wanted)

This all sounds like more steps forward than back to me (because at least what can be built is building). But if it's not good enough, I'm OK to cancel this pull request and let someone else find a better solution =)

[304NotModified]

I will merge this as it's better than nothing....

Thanks for your help! I will contact AppVeyor as I think to know a solution for the secure variables "leak" in PR's