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>