Added __FlashStringhelper* comparison operators to String object

Shouldn't you be using reinterpret_cast here? Please see: https://github.com/arduino/Arduino/pull/1770.

Note that PGM_P macro should be used for compatible with older version of gcc. Please see: https://github.com/arduino/Arduino/commit/98777e816f1dd2b86f8ee0d9fae66bdd54aeebd8 and https://github.com/arduino/Arduino/commit/ffddfc81f681bfbd81ebe4ec92c42b5a68cfccbf.