[Vm-dev] [OpenSmalltalk/opensmalltalk-vm] b4afea: Play the features.h dance

[Tobias Pape]

  Branch: refs/heads/Cog
  Home:   https://github.com/OpenSmalltalk/opensmalltalk-vm
  Commit: b4afeac5f51acafd24035e14dc370eb3954c57f1
      https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/b4afeac5f51acafd24035e14dc370eb3954c57f1
  Author: Tobias Pape <tobias at netshed.de>
  Date:   2021-03-25 (Thu, 25 Mar 2021)

  Changed paths:
    M platforms/unix/vm/include_ucontext.h

  Log Message:
  -----------
  Play the features.h dance

Since configure takes care that `_GNU_SOURCE` is there
whenever supported, _not_ having that means we cannot reasonably
ensure that we can access reg names, as defined via `__USE_GNU`.

The `features.h` makes sure that `__USE_GNU` is set if and only if
`_GNU_SOURCE` is set. So Bailing when not having it is the right choice.