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Some macros should be called only once, either because calling them
multiple time is unsafe, or because it is bad style. For instance
Autoconf ensures that AC_CANONICAL_BUILD and cousins
(see Canonicalizing) are evaluated only once, because it makes no
sense to run these expensive checks more than once. Such one-shot
macros can be defined using AC_DEFUN_ONCE.
Declare macro macro-name like
AC_DEFUNwould (see Macro Definitions), and emit a warning any time the macro is called more than once.
Obviously it is not sensible to evaluate a macro defined by
AC_DEFUN_ONCE in a macro defined by AC_DEFUN.
Most of the time you want to use AC_REQUIRE (see Prerequisite Macros).