Expand variables in user_options for boost_build

[petoknm]

For cross compiling (or other configuration) of boost, we often need to generate a user-config.jam file and pass the location of this file as user_options. That is why I needed variable expansion of user_options parameter of boost_build.

boost_build(
  ...
  user_options = [
    ...
    "--user-config=$(location :user_config)"
  ]
)

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