Add an Android version check for GNU strerror_r.

Summary:
Android didn't gain GNU's strerror_r until Marshmallow. If we're
building libc++ against something older (we build the NDK library
against the oldest release we support, currently Gingerbread), fall
back to the POSIX version.

Reviewers: mclow.lists, EricWF

Subscribers: tberghammer, danalbert, srhines, cfe-commits

Differential Revision: http://reviews.llvm.org/D21402

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272827 91177308-0d34-0410-b5e6-96231b3b80d8

src/system_error.cpp

@@ -21,6 +21,10 @@
 #include "string"
 #include "string.h"
 
+#if defined(__ANDROID__)
+#include <android/api-level.h>
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // class error_category
@@ -58,7 +62,8 @@ constexpr size_t strerror_buff_size = 1024;
 
 string do_strerror_r(int ev);
 
-#if defined(__linux__) && !defined(_LIBCPP_HAS_MUSL_LIBC)
+#if defined(__linux__) && !defined(_LIBCPP_HAS_MUSL_LIBC)                      \
+    && (!defined(__ANDROID__) || __ANDROID_API__ >= 23)
 // GNU Extended version
 string do_strerror_r(int ev) {
     char buffer[strerror_buff_size];