Adds if not defs around the flag declorations
Adds if not defs around the flag declorations

Checks to see if the varibles ARDUINO_C_FLAGS and ARDUINO_CXX_FLAGS has
already been set and only sets them if they are not. This fixes a bug
where the user could not define the flags manually as they where always
overwritten.

james147/ShrimpingItExamples,james147/ShrimpingItExamples