Improve CMake build

Consider the ful package delivers the two libraries wiringPi and
wiringPiDev with the associated binary gpio. We don't have to build and
install each component separately first.

By the way, set SOVERSION to version major. It also fixes wiringPiDev
shared library version not correctly set.

CMake configuration will fail if not found instead of waiting for build.

Building position independant code is the default behavior for CMake
when compiling shared library targets.

Added CMake to project so it can be cross-compiled

Spi

Remove gpio load/unload

release 3.4 - dev

Release 3.4

Gpioapi

Pwm

Warnings

Fixed warnings and removed unused function