Add support for target and prerequisite specific variable blocks

For example, now instead of:

lib{foo}: cxx.loptions += -static
lib{foo}: cxx.libs += -lpthread

We can write:

lib{foo}:
{
  cxx.loptions += -static
  cxx.libs += -lpthread
}

The same works for prerequisites as well as target type/patterns. For
example:

exe{*.test}:
{
  test = true
  install = false
}

1 files changed, 3 insertions, 0 deletions

diff --git a/build2/variable.hxx b/build2/variable.hxx
index e45914a..9688944 100644
--- a/build2/variable.hxx
+++ b/build2/variable.hxx
@@ -1227,6 +1227,9 @@ namespace build2
       size_t version = 0; // Incremented on each modification (variable_cache).
     };
 
+    // Note that we guarantee ascending iteration order (e.g., for predictable
+    // dump output in tests).
+    //
     using map_type = butl::prefix_map<reference_wrapper<const variable>,
                                       value_data,
                                       '.'>;