fix loop in autoconf volatile

[liumingc]

In recent macOS, when trying to autoconf, will show up the message and keep looping:

Your compiler chokes on the "volatile" modifier.
Never mind, we'll do without it.

But actually it's because clang will trigger errors like:

dblalign.c:22:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
   22 | main()

so i try to update the main function type spec.