#2 Raspberry Pi compatibility

[Div Slomin]

Here's a patch to get Bristol working on the Raspberry Pi. I've tested it successfully on the Raspberry Pi 3, but I think it applies to the older ones as well. The issues I addressed were:

• No support for SSE.
• The built-in sound chip is output-only, with no input port available. (Plugging in a USB sound card, etc. alleviates this, but it shouldn't be necessary for a soft-synth.)

The patch puts both of these under the control of enable switches in configure.