AC_CHECK_SIZEOF: Fix a bug, use standard default headers, and

AC_CHECK_SIZEOF: Fix a bug, use standard default headers, and
test.

* acgeneral.m4 (AC_CHECK_SIZEOF): Don't over quote $1, some []
were left in the C code, and had the macro fail.
Use AC_INCLUDES_DEFAULT.
* doc/autoconf.texi: Mention default includes.  Explain stdio.h
must always be given.

* tests/atspecific.m4 (TEST_MACRO): Updated the list of
exceptions.
(AT_CHECK_DEFINES): New macro, to check the content of config.h.
* tests/semantics.m4: All the tests are updated to use
AT_CHECK_DEFINES.
Test AC_CHECK_SIZEOF, AC_CHECK_HEADERS, and AC_CHECK_FUNCS.