Comment by cryptonector

Autoconf lets you check for all sorts of things:

  - instruction set architecture
  - OS versions
  - ABIs
  - libraries (whether they are installed)
    - and what functionality they provide
  - commands/executables
  - anything you can write a macro to check

All stuff too disparate to reliably have the OS be able to answer every question you might have about it and the stuff installed on it. You can't wait for any such system to learn how to answer the questions you might have about it, so some things you can only detect, either at build configuration time, build time, or run time.