Hello,

I'm trying to fix the appveyor setup for Boost.GIL, where I need to create a `user-config.jam` file with entries for third-party libraries libjpeg, libpng, libz, and libtiff.

While this isn't a problem with bash on travis-ci, I have a hard time doing the equivalent with appveyor, presumably because I'm unable to work out how this is spelled with cmd.exe or the PowerShell.

Here is my appveyor script:

https://github.com/boostorg/gil/blob/WIP/testing/.appveyor.yml#L82-L91

and here is the error this results in:

https://ci.appveyor.com/project/stefanseefeld/gil/build/1.0.109-develop/job/n6yco00602f8pqn7#L974

I'm sure I'm not the first one to attempt to do this (i.e., create a custom user-config.jam on appveyor). Can someone see what I'm doing wrong ?

Thanks,

-- 

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