Add support for directory prefixes

For example:

cxx{driver ../{foo bar}}
cxx{driver} ../cxx{foo bar}

1 files changed, 6 insertions, 0 deletions

diff --git a/build/path b/build/path
index 49d543c..ee7937b 100644
--- a/build/path
+++ b/build/path
@@ -181,6 +181,12 @@ namespace build
       init ();
     }
 
+    basic_path (const string_type& s, size_type p, size_type n)
+        : path_ (s, p, n)
+    {
+      init ();
+    }
+
     void
     swap (basic_path& p)
     {