Support boards with only SPI or Wire (I2C)

This library currently requires **both** `SPI` and `Wire` to be present for *any* target board. This can cause build failures for targets that only have one or the other. Ex:

https://github.com/adafruit/Adafruit_LiquidCrystal/pull/15
https://github.com/adafruit/Adafruit_Learning_System_Guides/pull/2340

Need to add some preproc or other logic to protect against this and allow these type of targets to be used.

Start by inspecting this library's SPI and Wire includes and compile-time checks, then reproduce the build with a target that provides only one of those interfaces. The work is done when targets with only SPI or only Wire build successfully while targets providing both continue to work.

Written by the indexing model from the issue text.