Why do we need to even run most of the things in ./configure? Why not just have a file in /etc which is updated when you install various packages which ./configure can read to learn various stats about the environment? Obviously it will still allow setting various things with parameters and create a Makefile, but much faster.

Keep in mind that the build intentionally depends on environment variables, people often install non-packaged dependencies in bad ways, and cross-compiling is a thing, so it's not that simple.

Some relevant discussion/pointers to other notes on this sort of proposal can be found here: https://utcc.utoronto.ca/~cks/space/blog/sysadmin/AutoconfVa...

(The conclusion I distilled out of reading that at the time, I think, was that this is actually sort of happening, but slowly, and autoconf is likely to stick around for a while, if only as a compatibility layer during the transition.)

That does already exist in GNU Autoconf: https://www.gnu.org/software/autoconf/manual/autoconf-2.65/h...

Not every OS is going to have such a file, and you also don't know if it matches the actual system ./configure runs on.