Wiringpi

This patch adds support for using `beep` on a Raspberry Pi, using an old BIOS speaker or piezo buzzer attached to its GPIO PWM pin.

When *not* running as root, ugly `system()` calls are used to call the `gpio` binary included with `wiringPi`. Note that this is significantly slower, as the binary needs to be called 5 times for each tone! You might be better off running `beep` as `suid`.

The original Makefile has been replaced with a Makefile.in to accommodate the conditional compilation with `wiringPi` support.

Build with `./configure && make`.