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.
Merge pull request #62 from james147/flags-fix

Adds if not defs around the flag declorations