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news/5009-use-conda-known-subdirs

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+### Enhancements
+
+* Use subdirs known to conda for selector definitions. (#5009)
+  This allows conda_build to support new architectures with just
+  a new version of conda. For new OSes, there are more information
+  needed for conda_build to work properly, including whether the
+  new OS is a UNIX-like platform, the shared library prefix, and
+  the binary archive format for the platform.
+
+### Bug fixes
+
+* <news item>
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>