diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -10,6 +10,11 @@
 #ifndef _LIBCPP_CONFIG
 #define _LIBCPP_CONFIG
 
+#if __has_include(<unistd.h>)
+// include unistd.h if available for some preprocessor defines like _NEWLIB_VERSION
+#  include <unistd.h>
+#endif
+
 #if defined(_MSC_VER) && !defined(__clang__)
 #  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #    define _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER