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It's our intent to add to config_args, not replace it, so invoke
config_args from builtin spackage.

- disable-spinlocks variant is only for @master, and @master built fine
  already without special rules

pkgconfig is a virtual provider in spack now

- now head-bedrock instead of develop-bedrock
- this prevents the package from triggering @develop versions on all
  dependencies because of the "develop" root name