config.guess: Avoid implicit int in definitions of main.

Implicit ‘int’ (e.g. ‘extern foo();’ meaning the same thing as
‘extern int foo();’) was dropped from the C standard in its 1999
edition.  Twenty-five years later, free C compilers are finally
starting to make this an error by default, so let’s not use it
anymore in config.guess probe programs.

(Note: As of this writing, GCC 14 and Clang 16 are both more lenient
for ‘main() { … }’ specifically than for other uses of implicit int.
Still, the writing is clearly on the wall.)

We continue to use ‘int main() { … }’, instead of ‘int main(void) { … }’,
because these programs may be compiled by truly ancient compilers that
do not recognize the keyword ‘void’.  This leaves open the possibility
of a compiler that errors by default on an empty argument list in a
function definition, which, prior to the 2024 C standard, is technically
still an “old-style” function definition; but we can worry about that
if and when it comes up.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Co-authored-by: Zack Weinberg <zack@owlfolio.org>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>

doc/config.guess.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man 1.48.5.
-.TH CONFIG.GUESS "1" "January 2024" "GNU config.guess (2024-01-01)" "User Commands"
+.TH CONFIG.GUESS "1" "May 2024" "GNU config.guess (2024-04-03)" "User Commands"
 .SH NAME
 config.guess \- guess the build system triplet
 .SH SYNOPSIS