Add support for specifying custom ad hoc pattern rule names

Besides diagnostics, this allows us to use name in the rule hint, for
example:

[rule_name=hello.link] exe{~'/(.*)/'}: obje{~'/\1/'}
{{
  $cxx.path -o $path($>) $path($<[0])
}}

[rule_hint=hello] exe{hello}: obje{hello}

obje{hello}: c{hello-c}

1 files changed, 8 insertions, 0 deletions

diff --git a/libbuild2/dump.cxx b/libbuild2/dump.cxx
index befd86b..14078da 100644
--- a/libbuild2/dump.cxx
+++ b/libbuild2/dump.cxx
@@ -225,6 +225,14 @@ namespace build2
     // Pattern.
     //
     os << ind;
+
+    // Avoid printing the derived name.
+    //
+    if (rp.rule_name.front () != '<' || rp.rule_name.back () != '>')
+    {
+      os << "[rule_name=" << rp.rule_name << "] ";
+    }
+
     rp.dump (os);
 
     // Recipes.