Windows build improvements

[zoltan151]

Modified the Makefile to properly handle building with OpenCL.

Updated the readme to include the commands to install the OpenCL libraries in mingw64, as the build will fail without these installed.

[NicolasFlamel1]

Thanks @zoltan151. I want to try to minimize the use of external dependencies not directly included in this project to simplify the build process for others. That's why I've directly included things like Apple's Metal C++ implementation and OpenCL libraries precompiled for the common Android ABIs in this repo.

MinGW should be able to link against the OpenCL DLL file included with Windows during the build process, C:\Windows\System32\OpenCL.dll, so I'd rather use that than require the user to download an external dependency if possible. What errors are you getting when trying link against that DLL file?

[zoltan151]

@NicolasFlamel1 It just didn't want to build - and turned out to be because I didn't have the OpenCL libraries installed. Doesn't seem that it ships out with Windows out-of-the-box. May need to rely on downloading that dependency, or packaging it into the project.