* doc/autoconf.texi: Use @acronym more consistently for acronyms

* doc/autoconf.texi: Use @acronym more consistently for acronyms
like BSD, GPL, LGPL.  Fix minor English typos.
(AC_STDC_HEADERS, AC_PROG_GCC_TRADITIONAL):
Mention that these macros are becoming obsolete.
(AC_STDC_HEADERS, AC_PROG_CC, AC_C_CONST, AC_C_VOLATILE):
Use more modern terminology for which standard is what.
(AC_PROG_CC): Mention gcc first, and remove obsolete references to egcs
and to ansi2knr.
(AC_PROG_CXX): Likewise.
(AC_C_PROTOTYPES, Test Functions, AC_LIBOBJ vs LIBOBJS):
Remove obsolete discussion about how to port to K&R.
(Guidelines for Test Programs): Suggest AC_HEADER_STDBOOL rather than
the obsolescent AC_HEADER_STDC.
(AC_FOO_IFELSE vs AC_TRY_FOO): Don't use #error; test programs
can't rely on it.