I would guess that the main cause of failures is autoconf, which has a habit of relying on extremely dubious C code to check for feature availability. On top of

I would guess that the main cause of failures is autoconf, which has a habit of relying on extremely dubious C code to check for feature availability. On top of that, the compiler runs are done in a way to guarantee that no one will see any warnings, even errors can tend to be silently ignored (!), and a particular version is often baked into a source package, so upgrading autoconf to a more sane version tends to take eons.