Generalize target/prerequisite var block, initial ad hoc target work

Target/prerequisite-specific variable blocks can now be present even if
there are prerequisites. For example, now instead of:

exe{foo}: cxx{foo}
exe{foo}: cc.loptions += ...

Or:

exe{foo}: cxx{foo}
exe{foo}:
{
  cc.loptions += ...
  cc.libs += ...
}

We can write:

exe{foo}: cxx{foo}
{
  cc.loptions += ...
  cc.libs += ...
}

This also works with dependency chains in which case the block applies
to the set of prerequisites (note: not targets) before the last ':'. For
example:

./: exe{foo}: libue{foo}: cxx{foo}
{
  bin.whole = false  # Applies to the libue{foo} prerequisite.
}

1 files changed, 3 insertions, 0 deletions

diff --git a/build2/token.cxx b/build2/token.cxx
index 81b7c55..8b62b46 100644
--- a/build2/token.cxx
+++ b/build2/token.cxx
@@ -36,6 +36,9 @@ namespace build2
     case token_type::lsbrace:        os << q << '['  << q; break;
     case token_type::rsbrace:        os << q << ']'  << q; break;
 
+    case token_type::labrace:        os << q << '<'  << q; break;
+    case token_type::rabrace:        os << q << '>'  << q; break;
+
     case token_type::assign:         os << q << '='  << q; break;
     case token_type::prepend:        os << q << "=+" << q; break;
     case token_type::append:         os << q << "+=" << q; break;