Make _enable_or_disable(...) return an empty array for conditional variants whose condition is not met (#27504)

lib/spack/docs/build_systems/autotoolspackage.rst

@@ -420,6 +420,24 @@ Or when one variant controls multiple flags:
    config_args += self.with_or_without('memchecker', variant='debug_tools')
    config_args += self.with_or_without('profiler', variant='debug_tools')
 
+
+""""""""""""""""""""
+Conditional variants
+""""""""""""""""""""
+
+When a variant is conditional and its condition is not met on the concrete spec, the
+``with_or_without`` and ``enable_or_disable`` methods will simply return an empty list.
+
+For example:
+
+.. code-block:: python
+
+   variant('profiler', when='@2.0:')
+   config_args += self.with_or_without('profiler)
+
+will neither add ``--with-profiler`` nor ``--without-profiler`` when the version is
+below ``2.0``.
+
 """"""""""""""""""""
 Activation overrides
 """"""""""""""""""""