the "build-to-host.m4" file seems to originally be from gnulib, and if that is installed on the system is not required by the build. So I see that as "part of the build system" myself.
I mean the github repository with exactly the same .ac and .am files works fine with local automake/autoconf generation without that file existing. And thus no backdoor (the test files are still there, but "harmless" without the initial kicking off point to actually de-obfuscate and include their contents)
Gnulib is not installable, it is meant to be copied (aka vendored) in the sources of the program that use it.
> if that is installed on the system is not required by the build
This specific file defines a macro that is used by autoconf, not by the build. If it is installed on the system it is not required by autoconf, but then gnulib is practically never installed.
Your original message blamed the backdoor on "the generated executable". This m4 file is not a generated file and not an executable. It is simply vendoring like you often see in other languages.
I think it was more "hiding" as vendored code rather than really being in that category. The git repo never contained that "vendoring", as the m4/gettext.m4 file doesn't exist autoreconf just copies one from a system store, (which on my machine never calls the tainted BUILD_TO_HOST macros in the first place, which also doesn't exist in the upstream xz git repo).
"Vendoring" by copying untracked files into the tarball seems discourteous to the definition. It seems to rely on the "possibly odd" behavior of autoreconf to allow files that happen to have the same name to override system-installed versions? I guess on the belief that local definitions can override them is useful? But that certainly bit them in the ass here. As to get a "completely" clean autoconf rebuild it looks like you have to delete matching files manually.
I mean the github repository with exactly the same .ac and .am files works fine with local automake/autoconf generation without that file existing. And thus no backdoor (the test files are still there, but "harmless" without the initial kicking off point to actually de-obfuscate and include their contents)