Add an Android version check for GNU strerror_r.

[danalbert]

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.

[EricWF]

LGTM.

I would have preferred to always use the POSIX version, but it's impossible to turn of GNU extensions without breaking the linux build.

src/system_error.cpp

 #include "cerrno"
 #include "cstring"
 #include "cstdio"
 #include "cstdlib"
 #include "cassert"
 #include "string"
 #include "string.h"
 
+#if defined(__ANDROID__)
+#include <android/api-level.h>
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 // class error_category
 
 error_category::error_category() _NOEXCEPT
 {
 }
 

 
 namespace {
 
 //  GLIBC also uses 1024 as the maximum buffer size internally.
 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];
     char* ret = ::strerror_r(ev, buffer, strerror_buff_size);
     return string(ret);
 }
 #else
 // POSIX version