Print warning when avrdude and avrdude.conf versions doesn't match

[MCUdude]

With this PR Avrdude will print a warning if the avrdude.conf file version doesn't match the avrdude executable version.

$ cat avrdude.conf | grep avrdude_conf_version
avrdude_conf_version = "7.1";

$ ./avrdude -cdryrun -patmega328p -PUSB 
avrdude warning: System wide configuration file version (7.1)
        does not match Avrdude build version (7.2-20230720)
avrdude: AVR device initialized and ready to accept instructions
avrdude: device signature = 0x1e950f (probably m328p)

avrdude done.  Thank you.

Resolves issue #1562

[mcuee]

Looks fine to me.

[stefanrueger]

I'd rather warn always if the version numbers do not coincide. Technically, AVRDUDE should be able to cope with older configuration files, but in practice you might expect malfunctions. So, I suggest to test str_eq() rather than str_starts().

[MCUdude]

I'd rather warn always if the version numbers do not coincide.

Sure! I've replaced str_starts with str_eq now