Node:Reporting Messages, Next:Dependencies Between Macros, Previous:Macro Names, Up:Writing Autoconf Macros
When macros statically diagnose abnormal situations, benign or fatal,
they should report them using these macros.  For dynamic issues, i.e.,
when configure is run, see Printing Messages.
| AC_DIAGNOSE (category, message) | Macro | 
| Report message as a warning (or as an error if requested by the
user) if it falls into the category.  You are encouraged to use
standard categories, which currently include: 
 | 
| AC_WARNING (message) | Macro | 
| Equivalent to AC_DIAGNOSE([syntax], message), but you are
strongly encouraged to use a finer grained category. | 
| AC_FATAL (message) | Macro | 
| Report a severe error message, and have autoconfdie. | 
When the user runs autoconf -W error, warnings from
AC_DIAGNOSE and AC_WARNING are reported as error, see
autoconf Invocation.