Add configure tests for C11-ish _Alignof and _Alignas.

These features are pretty feasible to implement usable macro-based
fallbacks on pre-C11 compilers.  Add some basic configure probes
to check these C11 features and AC_DEFINE reasonable fallbacks
where unsupported.

If Autoconf already detected C11 support, do nothing (as it tests
these features already).  Compilers which support these in C11 mode
usually also support them in other conformance modes, so it is still
useful to probe these specifically.