Move safe_strerror to anonymous namespace.

test/util.cc

@@ -37,3 +37,16 @@ void increment(char *s) {
     s[i] = '0';
   }
 }
+
+std::string get_system_error(int error_code) {
+#if defined(__MINGW32__) || !defined(_WIN32)
+  return strerror(error_code);
+#else
+  enum { BUFFER_SIZE = 200 };
+  char buffer[BUFFER_SIZE];
+  EXPECT_EQ(0, strerror_s(buffer, BUFFER_SIZE, error_code));
+  std::size_t max_len = BUFFER_SIZE - 1;
+  EXPECT_LT(std::strlen(buffer), max_len);
+  return buffer;
+#endif
+}