Avoid _Alignas/_Alignof redefinition on FreeBSD.

It seems that FreeBSD provides _Alignof and _Alignas macros in
various standard header files, including <stddef.h>.  Since the
configure probes don't include any headers, these are missed,
resulting in conflicting macro definitions.

To fix this, all we really need to do is include some header
file in the test program, so these system versions are visible
when testing for _Alignas and _Alignof support.