gcc/clang warning fixes and CMake implementation

[jwinarske]

This implements a CMake build file, and fixes all build warnings for GCC 7.5 and Clang 6.

Using the CMake file you can build the executable on Linux desktop in one step. It will pull down engine archive for linux x64. If libgpiod is not installed on host, it won't set the define to include it.

mkdir build && cd build
cmake .. -GNinja
autoninja

x64 Engine channel selection defaults to stable if none is selected, otherwise you switch channels like this

cmake .. -DCHANNEL=stable
cmake .. -DCHANNEL=beta
cmake .. -DCHANNEL=dev
cmake .. -DCHANNEL=master

Or build in visual code with the CMake Vector Of Bool extension.

Also I added a reference to your project in my flutter_embedded project

Cheers,
Joel

[ardera]

Thanks for your contribution!

There are a few things I'd change in the CMakeLists.txt though, so compilation on-target works as expected and the user has more freedom of choice for the engine used for compilation.

Other than that, looks great!

[ardera]

Okay, looks good. Merged it.
Thanks again!