Also, IIRC, many smaller projects using autoconf have copied in a bunch of boilerplate feature tests that they don't need, like "is strncpy available" "is snpri

Also, IIRC, many smaller projects using autoconf have copied in a bunch of boilerplate feature tests that they don't need, like "is strncpy available" "is snprintf available" "does realloc NULL work". As you mention, each one of these tests generates and compiles a minimal C file, and not in parallel. (And each test generates/defines a HAVE_SNPRINTF etc macro that your code can use to adapt based on available features. But if the project isn't as big as curl or git, it probably doesn't really adapt to all possible old and obscure systems anyway, so there's no point to 100s of such tests.)