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Some macros should be run before another macro if both are called, but neither requires that the other be called. For example, a macro that changes the behavior of the C compiler should be called before any macros that run the C compiler. Many of these dependencies are noted in the documentation.
Autoconf provides the AC_BEFORE macro to warn users when macros
with this kind of dependency appear out of order in a
`configure.ac' file. The warning occurs when creating
configure from `configure.ac', not when running
configure.
For example, AC_PROG_CPP checks whether the C compiler
can run the C preprocessor when given the @option{-E} option. It should
therefore be called after any macros that change which C compiler is
being used, such as AC_PROG_CC. So AC_PROG_CC contains:
AC_BEFORE([$0], [AC_PROG_CPP])dnl
This warns the user if a call to AC_PROG_CPP has already occurred
when AC_PROG_CC is called.
m4 print a warning message to the standard error output if
called-macro-name has already been called. this-macro-name
should be the name of the macro that is calling AC_BEFORE. The
macro called-macro-name must have been defined using
AC_DEFUN or else contain a call to AC_PROVIDE to indicate
that it has been called.
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