Index: libcxx/CMakeLists.txt
===================================================================
--- libcxx/CMakeLists.txt
+++ libcxx/CMakeLists.txt
@@ -242,6 +242,7 @@
   "Build libc++ with support for a monotonic clock.
    This option may only be set to OFF when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_HAS_MUSL_LIBC "Build libc++ with support for the Musl C library" OFF)
+option(LIBCXX_HAS_NEWLIB_LIBC "Build libc++ with support for the Newlib C library" OFF)
 option(LIBCXX_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXX_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXX_HAS_EXTERNAL_THREAD_API
@@ -845,6 +846,7 @@
 config_define_if(LIBCXX_HAS_WIN32_THREAD_API _LIBCPP_HAS_THREAD_API_WIN32)
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
+config_define_if(LIBCXX_HAS_NEWLIB_LIBC _LIBCPP_HAS_NEWLIB_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define_if(LIBCXX_ENABLE_PARALLEL_ALGORITHMS _LIBCPP_HAS_PARALLEL_ALGORITHMS)
 
Index: libcxx/include/__config
===================================================================
--- libcxx/include/__config
+++ libcxx/include/__config
@@ -327,6 +327,11 @@
 #  define _LIBCPP_USING_DEV_RANDOM
 #endif
 
+// Include newlib.h to get _NEWLIB_VERSION
+#if defined(_LIBCPP_HAS_NEWLIB_LIBC)
+#  include <newlib.h>
+#endif
+
 #if !defined(_LIBCPP_LITTLE_ENDIAN) && !defined(_LIBCPP_BIG_ENDIAN)
 #  include <endian.h>
 #  if __BYTE_ORDER == __LITTLE_ENDIAN
Index: libcxx/include/__config_site.in
===================================================================
--- libcxx/include/__config_site.in
+++ libcxx/include/__config_site.in
@@ -21,6 +21,7 @@
 #cmakedefine _LIBCPP_HAS_NO_MONOTONIC_CLOCK
 #cmakedefine _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
+#cmakedefine _LIBCPP_HAS_NEWLIB_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
 #cmakedefine _LIBCPP_HAS_THREAD_API_WIN32
Index: libcxx/test/support/nasty_macros.h
===================================================================
--- libcxx/test/support/nasty_macros.h
+++ libcxx/test/support/nasty_macros.h
@@ -60,7 +60,11 @@
 #endif
 
 #define __output NASTY_MACRO
+// The newlib uses __input in stdlib.h, so don't define it when newlib is
+// in use.
+#ifndef _LIBCPP_HAS_NEWLIB_LIBC
 #define __input NASTY_MACRO
+#endif
 
 #define __acquire NASTY_MACRO
 #define __release NASTY_MACRO