Implement support for dependency chains

Now instead of:

./: exe{foo}
exe{foo}: cxx{*}

We can write:

./: exe{foo}: cxx{*}

Or even:

./: exe{foo}: libue{foo}: cxx{*}

This can be combined with prerequisite-specific variables (which naturally
only apply to the last set of prerequisites in the chain):

./: exe{foo}: libue{foo}: bin.whole = false

1 files changed, 7 insertions, 2 deletions

diff --git a/build2/parser.hxx b/build2/parser.hxx
index dfb4c6b..582f925 100644
--- a/build2/parser.hxx
+++ b/build2/parser.hxx
@@ -67,15 +67,20 @@ namespace build2
 
     // If one is true then parse a single (logical) line (logical means it
     // can actually be several lines, e.g., an if-block). Return false if
-    // nothing has been parsed (i.e., we are on the same token).
+    // nothing has been parsed (i.e., we are still on the same token).
     //
-    // Note that after this function returns, the token is the first token on
+    // Note that after this function returns, the token is the first token of
     // the next line (or eos).
     //
     bool
     parse_clause (token&, token_type&, bool one = false);
 
     void
+    parse_dependency (token&, token_type&,
+                      names&&, const location&,
+                      names&&, const location&);
+
+    void
     parse_assert (token&, token_type&);
 
     void