Use std::stringstream instead of snprintf() for double->string conversion

`snprintf()` will use the current `LC_NUMERIC` locale
for converting a double to a string,
which will use a `,` instead of a `.` in some locales (e.g. de_DE).
`std::stringstream` allows setting the locale to `"C"` to always get a `.`.
This occurs only for that `stringstream` instance; no global is
altered.

src/lib_json/json_writer.cpp

@@ -63,22 +63,15 @@ std::string valueToString(UInt value) {
 #endif // # if defined(JSON_HAS_INT64)
 
 std::string valueToString(double value) {
-  // Allocate a buffer that is more than large enough to store the 16 digits of
-  // precision requested below.
-  char buffer[32];
-
-// Print into the buffer. We need not request the alternative representation
-// that always has a decimal point because JSON doesn't distingish the
-// concepts of reals and integers.
-#if defined(_MSC_VER) && defined(__STDC_SECURE_LIB__) // Use secure version with
-                                                      // visual studio 2005 to
-                                                      // avoid warning.
-  sprintf_s(buffer, sizeof(buffer), "%.16g", value);
-#else
-  snprintf(buffer, sizeof(buffer), "%.16g", value);
-#endif
-
-  return buffer;
+  // We need not request the alternative representation
+  // that always has a decimal point because JSON doesn't distingish the
+  // concepts of reals and integers.
+  std::stringstream str;
+  // Set locale to "C" to always get a '.' instead of a ','
+  str.imbue(std::locale::classic());
+  str.precision(16);
+  str << value;
+  return str.str();
 }
 
 std::string valueToString(bool value) { return value ? "true" : "false"; }