Add platform-conditional directive around spidev include

[alexeden]

Trying to install this package locally on a mac. Getting this error:

../src/spi.cc:1:10: fatal error: 'linux/spi/spidev.h' file not found
#include <linux/spi/spidev.h>
         ^~~~~~~~~~~~~~~~~~~~
1 error generated.

Makes sense, given that that library wouldn't exist on a mac. I noticed that in the pi-spi package, there's an #if __linux__ conditional around the inclusion of the SPI library. Would it make sense to include something like that in this package as well? Ideally I'd be able to run npm install anywhere without a breakdown.

[fivdi]

If spi-device is added to package.json in the optionalDependencies object rather than in the dependencies object then then it should be possible to achieve what you would like to achieve. There will be errors installing spi-device but npm will not terminate with an error while installing.

[alexeden]

Fair enough!