warning: unknown attribute 'error' ignored: __attribute__((error("")))

[jgfoster]

Code appears to compile correctly with the Arduino CLI compiler but not with the arduino_ci compiler. See https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Function-Attributes.html.

Changing cpp_library.rb:497 from -std=c++0x to -std=c++11 solves this problem. See Arduino-CI/arduino_ci#146.

Furthermore, this is only a warning and if there are no errors then it is silently ignored so we can proceed.