And in general, the build system of a large project is doing a lot of work and is considered pretty uninteresting and obscure. Random CMake macros or shell scripts would be just as likely to host bad code.

This is also why I like meson, because it's much more constrained than the others and the build system tends to be more modular and the complex parts split across multiple smaller, mostly independent scripts (written in Python or bash, 20-30 lines max). It's still complex, but I find it easier to organize.

Build systems can even have undefined behaviour in the C++ sense. For example Conan 2 has a whole page on that.

It's entirely possible they could have got that injection through review, even if they had that framwork and instead put it in source files used to generate autoconf soup.