revise check for stdalign.h to check if the keyword "alignas" works without stdalighn.h

[DrTimothyAldenDavis]

Hmmm...

I'm not sure with which compiler that user was seeing that problem. They don't seem to have given any hints about that.

Maybe, whichever compiler and which version of it they used, it defaulted to C23. And since in that version of C, align is a language keyword which doesn't require including any header, the implementers decided to remove stdalign.h entirely if that version of the standard is used.

If that is the actual problem, it might be better to check in the test whether using align requires the prior inclusion of any header.

Originally posted by @mmuetzel in #414 (comment)