Configuration and portability technology has evolved over the years.
Often better ways of solving a particular problem are developed, or
ad-hoc approaches are systematized. This process has occurred in many
parts of Autoconf. One result is that some of the macros are now
considered obsolete; they still work, but are no longer considered
the best thing to do, hence they should be replaced with more modern
macros. Ideally, autoupdate should replace the old macro calls
with their modern implementation.
Autoconf provides a simple means to obsolete a macro.
Define old-macro as implementation, just like
AC_DEFUN, but also declare old-macro to be obsolete.
When autoupdate is run, occurrences of old-macro will
be replaced by the text of implementation in the updated
configure.ac file.
If a simple textual replacement is not enough to finish the job of
updating a configure.ac to modern style, provide instructions for
whatever additional manual work is required as message. These
instructions will be printed by autoupdate, and embedded in the
updated configure.ac file, next to the text of implementation.
Normally, autoconf will also issue a warning (in the
“obsolete” category) when it expands old-macro. This warning
does not include message; it only advises the maintainer to run
autoupdate. If it is inappropriate to issue this warning, set
the silent argument to the word silent. One might want to
use a silent AU_DEFUN when old-macro is used in a
widely-distributed third-party macro. If that macro’s maintainers are
aware of the need to update their code, it’s unnecessary to nag all
of the transitive users of old-macro as well. This capability
was added to AU_DEFUN in Autoconf 2.70; older versions of
autoconf will ignore the silent argument and issue the warning
anyway.
Caution: If implementation contains M4 or M4sugar macros,
they will be evaluated when autoupdate is run, not emitted
verbatim like the rest of implementation. This cannot be avoided
with extra quotation, because then old-macro will not work when
it is called normally. See the definition of AC_FOREACH in
general.m4 for a workaround.
A shorthand version of AU_DEFUN, to be used when a macro has
simply been renamed. autoupdate will replace calls to
old-name with calls to new-name, keeping any arguments
intact. No instructions for additional manual work will be printed.
The silent argument works the same as the silent argument
to AU_DEFUN. It was added to AU_ALIAS in Autoconf 2.70.
Caution: AU_ALIAS cannot be used when new-name is
an M4 or M4sugar macro. See above.