AC_PROG_CXX, AC_PROG_CXXCPP, AC_REQUIRE_CPP
for checking both C++ and C features in one configure script.
* New macros AC_CHECKING, AC_VERBOSE, AC_WARN, AC_ERROR for printing messages.
-* New macros AC_FIND_XTRA, AC_MMAP, AC_SIZEOF_TYPE.
+* New macros AC_FIND_XTRA, AC_MMAP, AC_SIZEOF_TYPE, AC_PREREQ.
* AC_FIND_X looks for X in more places.
* AC_PROG_INSTALL defaults to install.sh instead of cp, if it's in srcdir.
install.sh is distributed with Autoconf.
* Relative srcdir values are not made absolute.
* configure accepts --help, --version, --silent/--quiet options.
* config.status accepts --help, --version options.
+* Autoconf library files are installed in ${datadir}/autoconf, not ${datadir}.
* Namespace cleanup: all shell variables used internally by Autoconf
have names beginning with `ac_'.
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