Update to compile with newer avr-gcc versions.

[dietervansteenwegen]

Interrupt vector names have been changed since the last commit of this code.

This page shows the new names.

Additionally, the SIGNAL() macro is deprecated and should be replaced with ISR(). See this note.

And newer versions of the avr-gcc compiler require that variables in PROGMEM are declared as const (see the GCC docs).

[dietervansteenwegen]

Some additional information: I wanted to add an hourly beep (during office hours) option to the code.

I couldn't not get the gcc compiler to code as it was. These are the minimum changes to update it to new requirements/interrupt vector names.

Yes, some people are still using this clock :)