Fix issue with fetching packages when doing cross-compilation

[bionicbeagle]

This change changes the behavior with cross compilation when using packages. Previously the package would not properly fetch or install the cross-compiled packages.

Fixes #4935

[waruqi]

It will break something, It may introduce more host libraries from on_fetch as well as pkg-config, causing linking errors. Only vcpkg/conan supports fetching cross-compiled libraries.

If we want to improve this, we need to consider more details.

[bionicbeagle]

I see. I guess I would consider making conan/vcpkg functional an improvement over the current behaviour which is a very obscure failure mode with every package manager.

I'm not sufficiently familiar with the xmake internals to propose a solution but it seems like it would be useful to have a way to discriminate between the different package managers wrt their cross-compilation capabilities so that a better error could be produced in the cases where it is not supported, and let the package managers which support cross-compilation do their thing.

[waruqi]

I will try to improve it, but it may take some time.

[bionicbeagle]

Thanks. I'm wondering though if it would be valuable to at least make vcpkg/conan work in the meantime? As far as I understand the net impact of this change will be:

• xrepo/vcpkg/conan will work for cross-compilation just as it used to
• other package managers may fail in some manner

whereas the current code leaves us with:

• xrepo works for cross-compilation
• all other package managers end up with the obscure error in Unable to use asio vcpkg dependency for MacOS (cross compilation) with xmake version later than 2.8.2 #4935, even if they are able to handle cross-compilation

with this change we are able to build for MacOS with latest xmake whereas without it we have to remain on 2.8.2 which is not ideal as we'd really like to leverage newer functionality.

[waruqi]

Thanks. I'm wondering though if it would be valuable to at least make vcpkg/conan work in the meantime? As far as I understand the net impact of this change will be:

• xrepo/vcpkg/conan will work for cross-compilation just as it used to
• other package managers may fail in some manner

No, the impact of the current patch may be a bit severe, and more users will fail to compile due to finding the wrong system libraries from pkg-config than vcpkg/conan.

[waruqi]

try #4963