When macros statically diagnose abnormal situations, benign or fatal, it is possible to make autoconf detect the problem, and refuse to create configure in the case of an error. The macros in this section are considered obsolescent, and new code should use M4sugar macros for this purpose, see Diagnostic Macros.
On the other hand, it is possible to want to detect errors when configure is run, which are dependent on the environment of the user rather than the maintainer. For dynamic diagnostics, see Printing Messages.
Report message as a warning (or as an error if requested by the user) if warnings of the category are turned on. This macro is obsolescent; you are encouraged to use:
m4_warn([category], [message])instead. See m4_warn, for more details, including valid category names.
Report message as a syntax warning. This macro is obsolescent; you are encouraged to use:
m4_warn([syntax], [message])instead. See m4_warn, for more details, as well as better finer-grained categories of warnings (not all problems have to do with syntax).
Report a severe error message, and have autoconf die. This macro is obsolescent; you are encouraged to use:
m4_fatal([message])instead. See m4_fatal, for more details.
When the user runs ‘autoconf -W error’, warnings from
m4_warn (including those issued through AC_DIAGNOSE and
AC_WARNING) are reported as errors, see autoconf Invocation.