acinclude: Fix -Wstrict-prototypes and -Wold-style-definition detection.
AC_LANG_PROGRAM(,) uses a program like this:
int main() { return 0; }
but that triggers warnings for -Wstrict-prototypes and for
-Wold-style-definition, since this definition of main() lacks a prototype
and is therefore old-style. This meant that -Wstrict-prototypes and
-Wold-style-definition weren't being turned on for new-enough GCC. This
commit fixes the problem by changing the program that is test-compiled to:
int x;
which doesn't make any compilers mad, as far as I know.
I recently upgraded to GCC 6.1 and just now noticed the issue, so I think
that GCC somewhere between version 4.9 and version 6.1 must have started
warning about main() when it's declared this way.
Also, fix a few functions that lacked prototypes.
Signed-off-by: Ben Pfaff <blp@ovn.org> Acked-by: Andy Zhou <azhou@ovn.org>