Tighten up attribute recognition during parsing

Now it should be possible to use `[]` for wildcard patterns, for example:

  foo = foo.[hit]xx

Note that a leading bracket expression will still be recognized as attributes
and escaping or quoting it will inhibit pattern matching. To resolve this case
we need to specify an empty attribute list:

  foo = [] [abc]-foo.cxx

1 files changed, 2 insertions, 2 deletions

diff --git a/old-tests/attribute/buildfile b/old-tests/attribute/buildfile
index 7338641..a9abf24 100644
--- a/old-tests/attribute/buildfile
+++ b/old-tests/attribute/buildfile
@@ -6,10 +6,10 @@
 #[foo=dir/file{bar}]  # error: invalid attribute key
 
 #[foo] print hello    # error: attributes before print
-#[foo]./              # error: attributes before directory scope
+#[foo] ./             # error: attributes before directory scope
 #{
 #}
-#[foo]./:             # error: attributes before target scope
+#[foo] ./:            # error: attributes before target scope
 #./: [foo] buildfile  # error: attributes before prerequisites
 #import [foo] libz    # error: attributes without variable