Change gdb/common/ to use Automake

[Tom Tromey]

Pedro> As long as we have auto-dependencies, it's cool with me.  I'd
Pedro> love to have them on gdbserver too.

I started converting gdbserver to automake today, but it was taking too
long and I have too much else to do.  I'm happy to send my starter
Makefile.am plus some comments to anybody who wants to pick it up,
though.  It shouldn't take too awfully long for someone to finish.

Tom