There are two different builtin conditionals in m4. The first is
ifdef:
ifdef(name, string-1, opt string-2)
which makes it possible to test whether a macro is defined or not. If
name is defined as a macro, ifdef expands to
string-1, otherwise to string-2. If string-2 is
omitted, it is taken to be the empty string (according to the normal
rules).
ifdef(`foo', ``foo' is defined', ``foo' is not defined') =>foo is not defined define(`foo', `') => ifdef(`foo', ``foo' is defined', ``foo' is not defined') =>foo is defined
The macro ifdef is recognized only with parameters.
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