* doc/autoconf.texi (Generic Types): Document the restrictions

* doc/autoconf.texi (Generic Types): Document the restrictions
on types imposed by AC_CHECK_TYPE, AC_CHECK_TYPES.
(Generic Compiler Characteristics): AC_CHECK_SIZEOF now works
with objects too.  Document the restrictions on its use.
Document the restrictions on AC_CHECK_ALIGNOF's type argument.
* lib/autoconf/types.m4 (_AC_CHECK_TYPE_NEW):
For C, just try sizeof (TYPE) and sizeof ((TYPE)); if the former
works but the latter doesn't, then it's a valid type.
This lets people use function types and so forth.
For C++ there doesn't seem to be a simple solution, so leave it alone.
(AC_CHECK_SIZEOF): Allow argument to be a variable.
(AC_CHECK_SIZEOF, AC_CHECK_ALIGNOF): Don't bother to invoke
AC_CHECK_TYPE; that wasn't documented or necessary.