Fix macOS <-> Linux cross cflags resolution

[sfackler]

Changelog: Fix: Fix flags passed by AutotoolsToolchain when cross compiling from macOS to a non-Apple OS.
Docs: Omit

Fixes #13219

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[sfackler]

ping

[jcar87]

Hi @sfackler, thanks so much for your detailed report and for opening this PR.

Indeed I believe the logic was not robust enough to account for the fact that you can target Linux from macOS.

I have made a small fix to your PR - I believe the conditional should cover the case where os_build is MacOS, and the target OS is one of the Apple operating systems.

• The call to apple_sdk_path() has a call to xcrun, which is a utility only available on macOS - thus, the conditional should cover that case.
• The flags -isysroot (specifically pointing to an Apple-provided SDK) and -arch carry two implications: that the sysroot is an Apple provided SDK (and thus, the os_host is an Apple one), and that the compiler accepts the -arch flag (clang or AppleClang, although we are not specifically checking for this condition).

Note for @czoido: there's a chance this doesn't fully cover the case of targetting iOS from Linux (see https://github.com/tpoechtrager/cctools-port) - but this PR at least doesn't prevent that :D

[memsharded]

It would be great to add a test for this.