Add x86_64 as allowed value for CMAKE_SYSTEM_PROCESSOR

[tobiasleibner]

This is needed for me to compile the libjpeg-turbo port.

[MarkSchofield]

Using x86_64 for CMAKE_SYSTEM_PROCESSOR has been brought-up before: #130. As per comments there;

The CMAKE_SYSTEM_PROCESSOR documentation suggests that it's values should align with values of CMAKE_HOST_SYSTEM_PROCESSOR, which - on Windows - is set to 'the value of the environment variable PROCESSOR_ARCHITECTURE'. The PROCESSOR_ARCHITECTURE environment variable isn't ever x86_64.

It doesn't seem to be a valid value....?

Regarding, "building the libjpeg-turbo port". You shouldn't need a toolchain to have VCPkg build a port - VCPkg hooks up it's own 'toolchain'.

Can you provide a few more details as to what you're trying to do?

[MarkSchofield]

TBH, it looks like https://github.com/libjpeg-turbo/libjpeg-turbo accommodates AMD64 as a value for CMAKE_SYSTEM_PROCESSOR - in CMakeLists.txt there's logic to lowercase CMAKE_SYSTEM_PROCESSOR and then it's compared with amd64.

If you're trying to use https://github.com/MarkSchofield/WindowsToolchain to compile VCPkg ports, I had a write-up over in the comments for #116. Whilst you're supposed to be able to provide a toolchain to build VCPkg ports, there's still assumptions in the VCPkg infrastructure itself that environment variables are set, and the toolchains that VCPkg hooks-up can't set environment variables. It seems like filing VCPkg bugs for the assumptions would be good, I just haven't had time.