Arduino 1.5.8 beta fixup

Code changes to compile dcf77_library in Arduino Version 1.5.8 Beta (AVR toolchain: gcc 4.8.1, avr-libc 1.8.0).

Code successfully compiled on Windows 7 (x64) and Linux (x64).
Tested on an ATmega328P (Arduino Uno compatible board), got a sync to a DCF-77 signal after several minutes.

About commit 4871693: For some reason the gcc compiler crashed during compiling of the library as long as the "always_inline" attribute was used for the "uint8_t days_per_month()" function.
I don't know too much about the inner workings of the library, but i suspect removing the "always_inline" attribute should not have too much of a negative effect on the decoding.