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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 Getting the Canonical System Type) 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_DEFUN would (see Macro Definitions), but add additional logic that guarantees that only the
first use of the macro (whether by direct expansion or
AC_REQUIRE) causes an expansion of macro-body; the
expansion will occur before the start of any enclosing macro defined by
AC_DEFUN. Subsequent expansions are silently ignored.
Generally, it does not make sense for macro-body to use parameters
such as $1.
Prior to Autoconf 2.64, a macro defined by AC_DEFUN_ONCE would
emit a warning if it was directly expanded a second time, so for
portability, it is better to use AC_REQUIRE than direct
invocation of macro-name inside a macro defined by AC_DEFUN
(see Prerequisite Macros).