[vcpkg/host-triplet] Make the host triplet selection better

[Neumann-A]

Problem: x64-windows-static is not a host triplet on x64 & windows
Observed while by trying to make qtbase depend on itself for the host.

vcpkg's selection of the host triplet is quite rudimentary in terms of the host triplet selection. This affects all custom triplets!

Suggestion:
Look into the target triplet (VCPKG_SYSTEM_NAME & VCPKG_TARGET_ARCHITECTURE) and compare the values against the host.

• If the values match the host use the target triplet as the host triplet.
• if not: do as before.
• if user passes: --host-triplet. Don't do anything and just use the provided value (current behavior)

cc @ras0219-msft @strega-nil

other observation: Compiler detection seems to run with host = target triplet according to --debug (Intended or bug?)

[Hoikas]

FWIW, if you need a host triplet copy of python3 and vcpkg selects x64-windows-static instead of x64-windows, then the built python.exe will be unable to load extension modules.

[Neumann-A]

Isn't python handled by vcpkg_find_acquire_program any way ?

[Hoikas]

Isn't python handled by vcpkg_find_acquire_program any way ?

At this time, yes. However, if you want a functioning Python 3 interpreter on Windows 7 for your portfile, then you would need to install python3[deprecated-win7-support] for the host. Currently, no one does that, but I think it's worth keeping in mind that some ports have major caveats with certain triplets.

[autoantwort]

This only works if people don't set VCPKG_SYSTEM_NAME & VCPKG_TARGET_ARCHITECTURE depending on the port name (afaik currently allowed)