Cherry-pick upstream r252457

include/__config

 // -*- C++ -*-
 //===--------------------------- __config ---------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is dual licensed under the MIT and the University of Illinois Open
 // Source Licenses. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
@@ skipping 100 matching lines @@
 # endif
 #endif // __sun__
 
 #if defined(__native_client__)
   // NaCl's sandbox (which PNaCl also runs in) doesn't allow filesystem access,
   // including accesses to the special files under /dev. C++11's
   // std::random_device is instead exposed through a NaCl syscall.
 # define _LIBCPP_USING_NACL_RANDOM
 #endif // defined(__native_client__)
 
+#ifdef __musl__
+#define _LIBCPP_HAS_MUSL_LIBC
+#endif // defined(__musl__)
+
 #if !defined(_LIBCPP_LITTLE_ENDIAN) || !defined(_LIBCPP_BIG_ENDIAN)
 # include <endian.h>
 # if __BYTE_ORDER == __LITTLE_ENDIAN
 #  define _LIBCPP_LITTLE_ENDIAN 1
 #  define _LIBCPP_BIG_ENDIAN    0
 # elif __BYTE_ORDER == __BIG_ENDIAN
 #  define _LIBCPP_LITTLE_ENDIAN 0
 #  define _LIBCPP_BIG_ENDIAN    1
 # else  // __BYTE_ORDER == __BIG_ENDIAN
 #  error unable to determine endian
@@ skipping 194 matching lines @@
 #define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
 #endif
 
 #if __ISO_C_VISIBLE >= 2011 || __cplusplus >= 201103L
 #if defined(__FreeBSD__)
 #define _LIBCPP_HAS_QUICK_EXIT
 #define _LIBCPP_HAS_C11_FEATURES
 #elif defined(__ANDROID__)
 #define _LIBCPP_HAS_QUICK_EXIT
 #elif defined(__linux__)
-#include <features.h>
+#if !defined(_LIBCPP_HAS_MUSL_LIBC)
+# include <features.h>
 #if __GLIBC_PREREQ(2, 15)
 #define _LIBCPP_HAS_QUICK_EXIT
 #endif
 #if __GLIBC_PREREQ(2, 17)
 #define _LIBCPP_HAS_C11_FEATURES
 #endif
+#else // defined(_LIBCPP_HAS_MUSL_LIBC)
+#define _LIBCPP_HAS_QUICK_EXIT
+#define _LIBCPP_HAS_C11_FEATURES
 #endif
+#endif // __linux__
 #endif
 
 #if (__has_feature(cxx_noexcept))
 #  define _NOEXCEPT noexcept
 #  define _NOEXCEPT_(x) noexcept(x)
 #  define _NOEXCEPT_OR_FALSE(x) noexcept(x)
 #else
 #  define _NOEXCEPT throw()
 #  define _NOEXCEPT_(x)
 #  define _NOEXCEPT_OR_FALSE(x) false
@@ skipping 358 matching lines @@
 #  elif (defined(_MSC_VER) && !defined(__clang__)) && !defined(_CPPRTTI)
 #    define _LIBCPP_NO_RTTI
 #  endif
 #endif
 
 #ifndef _LIBCPP_WEAK
 #  define _LIBCPP_WEAK __attribute__((__weak__))
 #endif
 
 #endif  // _LIBCPP_CONFIG