Referencing environ needs extern on BSD.

See e.g., LibcOverlayShims.h in Swift.

Author: 3405691582

CoreFoundation/Base.subproj/CFPlatform.c

@@ -1630,6 +1630,9 @@ CF_EXPORT char **_CFEnviron(void) {
 #elif TARGET_OS_WIN32
     return _environ;
 #else
+#if TARGET_OS_BSD
+    extern char **environ;
+#endif
     return environ;
 #endif
 }