A few kinds of features can't be guessed automatically by running test
programs. For example, the details of the object-file format, or
special options that need to be passed to the compiler or linker. You
can check for such features using ad-hoc means, such as having
configure check the output of the
uname program, or
looking for libraries that are unique to particular systems. However,
Autoconf provides a uniform method for handling unguessable features.