make: fix wrong expression evaluation in -dL mode

The modifier ':C' now only compiles the regular expression if the result
of the expression is actually needed.

Several other modifiers have the same bug of evaluating the expression
in cases where this is not needed.  It just doesn't show up because they
don't have any noticeable side effects, other than wasting CPU time.
This affects irrelevant conditions as well.

(rillig)