[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[via llvm-commits]

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@@ -0,0 +1,20 @@
+//===-- Common defines for sharing LLVM libc with LLVM projects -*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_LIBC_COMMON_H
+#define LLVM_LIBC_SHARED_LIBC_COMMON_H
+
+// Use system errno.
+#undef LIBC_ERRNO_MODE
+#define LIBC_ERRNO_MODE LIBC_ERRNO_MODE_SYSTEM
+
+#ifndef LIBC_NAMESPACE
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lntue wrote:

That will break users of libc++ (and LLVM in the future) who are non-users of LLVM libc and do not use cmake build.  Since these are only internal used for LLVM projects, we can dictate a safe default config here without relying on any build system.  It will also allow us to freely change this later if we want to, without breaking downstream users of LLVM and libc++, as long as we can make sure that the change is transient.

https://github.com/llvm/llvm-project/pull/143187