Cherry-pick upstream r252457

include/__config

Index: include/__config
diff --git a/include/__config b/include/__config
index 6f57df5d233f7c3ae673727eb92ccb14443278ef..15af1598d23a446dea297f6eee37ef6861b6fbf1 100644
--- a/include/__config
+++ b/include/__config
@@ -118,6 +118,10 @@
 # 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
@@ -332,14 +336,19 @@ typedef __char32_t char32_t;
 #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))