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, 1 insertions, 0 deletions

diff --git a/build2/prerequisite.hxx b/build2/prerequisite.hxx
index 0f9a9cd..c584e19 100644
--- a/build2/prerequisite.hxx
+++ b/build2/prerequisite.hxx
@@ -83,6 +83,7 @@ namespace build2
     // Prerequisite-specific variables.
     //
     // Note that the lookup is often ad hoc (see bin.whole as an example).
+    // But see also parser::lookup_variable() if adding something here.
     //
   public:
     variable_map vars;