Make fsdir{} usable as target of ad hoc Buildscript recipes

In particular, it can now be used to represent a directory symlink. For
example:

exe{hello}: ... fsdir{assets}

fsdir{assets}:
% update
{{
  ln -s $src_base/assets $out_base/assets
}}
% clean
{{
  rm $out_base/assets
}}

1 files changed, 8 insertions, 0 deletions

diff --git a/libbuild2/build/script/runner.cxx b/libbuild2/build/script/runner.cxx
index e08ebbf..5d9764b 100644
--- a/libbuild2/build/script/runner.cxx
+++ b/libbuild2/build/script/runner.cxx
@@ -43,6 +43,14 @@ namespace build2
               }
             }
           }
+          else if (const fsdir* fd = env.target.is_a<fsdir> ())
+          {
+            // Compare ignoring the trailing directory separator.
+            //
+            if (path_traits::compare (i->path.string (),
+                                      fd->dir.string ()) == 0)
+              m = fd;
+          }
           else
           {
             for (m = &env.target; m != nullptr; m = m->adhoc_member)