We aim to compile on C89, but we try to avoid using any feature that's
not in even older standards if possible. As this is a configure system, it
should not need yet another configure system to compile. In this document
we explain a few design decisions related to this topic.

alloca: only a few systems supports alloca, we use (the slower) malloc/free
method to get local storage.

inline: do not use inline functions, some older C compilers don't like it
(for example OSF/1 (Tru64)).

local functions: for similar reason to that discussed at inline, we never
use local functions (function definition inside a function).

strdup(): POSIX.1-2001; we use strclone() instead

snprintf(): C99; we use less safe sprintf() with "big enough" buffers instead.

system(): popen() and pipe() are defined in POSIX.1-2001 only so we need
to use the slower method by running system() and redirect output to a file
that we later read.

vararg macros: never; use <stdarg.h> instead.