Implement new potential directive keyword test

Now we can use directive names as variables and targets type, for
example:

print = foo # variable
print{foo}: # target

1 files changed, 5 insertions, 0 deletions

diff --git a/build/parser b/build/parser
index ca27890..f3f88d5 100644
--- a/build/parser
+++ b/build/parser
@@ -128,6 +128,11 @@ namespace build
     void
     skip_block (token&, token_type&);
 
+    // Return true if the name token can be considered a directive keyword.
+    //
+    bool
+    keyword (token&);
+
     // Buildspec.
     //
     buildspec