Various fixes to make ViennaCLBench buildable in 2022.

[illwieckz]

• Fix Qt5 compilation, some deprecated Qt5 components may still be used but it works.
• New way to build optional external libarchive.
• Drop Qt4, it's almost impossible to use today (to build Qt4 today one has to build it with GCC 7 and build GCC 7 with GCC 10 and…).
• Drop platform icons, they can't be right.
• Update README.md file.

About icons, they can't be right. The code displays everything it didn't know at the time of 2015 as Nvidia, and does the guess based on platform, but there are more than one platform per GPU brand (AMD had fglrx, Orca, PAL, ROCm and mesa Clover, Intel had Beignet, Neo and many others, etc.) and there are more than one GPU brand per platform (Mesa Clover supports Nvidia and AMD, Mesa rusticl is expected to support Nvidia, AMD and Intel), and some are GPU agnostic (clvk is expected to support all Vulkan devices whatever the brand). Also the future can bring more OpenCL implementations no one can predict. So I decided to remove the GPU images and just print the platform name instead, this string being made up from driver information it can't be wrong.

[karlrupp]

Excellent, @illwieckz , this is fantastic. I'll test the changes and then report back. Might take a bit, so please be patient :-)