configure.ac: update main() signatures to conform to the standard

[orlitzky]

There are some tests in configure.ac that contain,

int main() { ... }

That's not the correct signature for main() according to the C standard, and newer compilers are going to reject it. More information about this can be found at,

https://wiki.gentoo.org/wiki/Modern_C_porting

In this case, the fix is simply to write

int main(int argc, char** argv) { ... }

instead.