Add config.bin.target var/hint, use to decide libso{} install mode

Normally the user doesn't need to specify config.bin.target explicitly
since the cxx module will hint it. We now also have the whole set of
target's components:

bin.target.{cpu,vendor,system,version,class}

1 files changed, 15 insertions, 0 deletions

diff --git a/build2/config/utility b/build2/config/utility
index beab758..c47cecf 100644
--- a/build2/config/utility
+++ b/build2/config/utility
@@ -55,6 +55,21 @@ namespace build2
       return required (root, name, string (default_value), override);
     }
 
+    // As above, but leave the unspecified value as undefined (and return
+    // NULL pointer) rather than setting it to the default value.
+    //
+    // This can be useful when we don't have a default value but may figure
+    // out some fallback. See config.bin.target for an example.
+    //
+    pair<const value*, bool>
+    required (scope& root, const variable&);
+
+    inline pair<const value*, bool>
+    required (scope& root, const string& name)
+    {
+      return required (root, var_pool.find (name));
+    }
+
     // Set, if necessary, an optional config.* variable. In particular,
     // an unspecified variable is set to NULL which is used to distinguish
     // between the "configured as unspecified" and "not yet configured"