Enable $-substitution in %define values.

doc/ZConfig/zconfig.tex

@@ -225,8 +225,9 @@ Configuration values which include a \character{\$} as part of the
 actual value will need to use \code{\$\$} to get a single
 \character{\$} in the result.
 
-The values of defined names are not processed in any way, and may not
-contain references to named definitions.
+The values of defined names are processed in the same way as
+configuration values, and may contain references to named
+definitions.
 
 For example, the value for the \code{key} will evaluate to
 \code{value}: