Fix build on Android NDK

where XSI-compliant version of strerror_r is used violating the docs at
http://linux.die.net/man/3/strerror_r

format.cc

@@ -134,7 +134,13 @@ int safe_strerror(
     int error_code, char *&buffer, std::size_t buffer_size) FMT_NOEXCEPT(true) {
   assert(buffer != 0 && buffer_size != 0);
   int result = 0;
-#ifdef _GNU_SOURCE
+#if ((_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && !_GNU_SOURCE) || __ANDROID__
+  // XSI-compliant version of strerror_r.
+  result = strerror_r(error_code, buffer, buffer_size);
+  if (result != 0)
+    result = errno;
+#elif _GNU_SOURCE
+  // GNU-specific version of strerror_r.
   char *message = strerror_r(error_code, buffer, buffer_size);
   // If the buffer is full then the message is probably truncated.
   if (message == buffer && strlen(buffer) == buffer_size - 1)