Add update operation-specific variable with unmatch|match additional values

Note that the unmatch (match but do not update) and match (update during
match) values are only supported by certain rules (and potentially only for
certain prerequisite types).

Additionally:

- All operation-specific variables are now checked for false as an override
  for the prerequisite-specific include value. In particular, this can now be
  used to disable a prerequisite for update, for example:

  ./: exe{test}: update = false

- The cc::link_rule now supports the update=match value for headers and ad hoc
  prerequisites. In particular, this can be used to make sure all the library
  headers are updated before matching any of its (or dependent's) object
  files.

1 files changed, 6 insertions, 3 deletions

diff --git a/libbuild2/dyndep.hxx b/libbuild2/dyndep.hxx
index 257ef7b..6632eb6 100644
--- a/libbuild2/dyndep.hxx
+++ b/libbuild2/dyndep.hxx
@@ -22,9 +22,12 @@ namespace build2
   class LIBBUILD2_SYMEXPORT dyndep_rule
   {
   public:
-    // Update the target during the match phase. Return true if it has changed
-    // or if the passed timestamp is not timestamp_unknown and is older than
-    // the target.
+    // Update the target during the match phase. Return true if the target has
+    // changed or, if the passed timestamp is not timestamp_unknown, it is
+    // older than the target.
+    //
+    // Note that such a target must still be updated during the execute phase
+    // in order to keep the dependency counts straight.
     //
     static bool
     update (tracer&, action, const target&, timestamp);