Implement ad hoc regex pattern rule support

An ad hoc pattern rule consists of a pattern that mimics a dependency
declaration followed by one or more recipes. For example:

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

If a pattern matches a dependency declaration of a target, then the recipe is
used to perform the corresponding operation on this target. For example, the
following dependency declaration matches the above pattern which means the
rule's recipe will be used to update this target:

exe{hello}: cxx{hello}

While the following declarations do not match the above pattern:

exe{hello}:   c{hello}  # Type mismatch.
exe{hello}: cxx{howdy}  # Name mismatch.

On the left hand side of `:` in the pattern we can have a single target or an
ad hoc target group. The single target or the first (primary) ad hoc group
member must be a regex pattern (~). The rest of the ad hoc group members can
be patterns or substitutions (^). For example:

<exe{~'/(.*)/'} file{^'/\1.map/'}>: cxx{~'/\1/'}
{{
  $cxx.path -o $path($>[0]) "-Wl,-Map=$path($>[1])" $path($<[0])
}}

On the left hand side of `:` in the pattern we have prerequisites which can
be patterns, substitutions, or non-patterns. For example:

<exe{~'/(.*)/'} file{^'/\1.map/'}>: cxx{~'/\1/'} hxx{^'/\1/'} hxx{common}
{{
  $cxx.path -o $path($>[0]) "-Wl,-Map=$path($>[1])" $path($<[0])
}}

Substitutions on the left hand side of `:` and substitutions and non-patterns
on the right hand side are added to the dependency declaration. For example,
given the above rule and dependency declaration, the effective dependency is
going to be:

<exe{hello} file{hello.map>: cxx{hello} hxx{hello} hxx{common}

1 files changed, 12 insertions, 8 deletions

diff --git a/libbuild2/scope.hxx b/libbuild2/scope.hxx
index 3529495..b83f699 100644
--- a/libbuild2/scope.hxx
+++ b/libbuild2/scope.hxx
@@ -338,10 +338,13 @@ namespace build2
     // extensions, special names (e.g., '.' and '..'), or anything else that
     // might be relevant. Process the name (in place) by extracting (and
     // returning) extension, adjusting dir/leaf, etc., (note that the dir is
-    // not necessarily normalized). Return NULL if not found.
+    // not necessarily normalized). If the target type is already resolved,
+    // then it can be passed as the last argument. Return NULL if not found.
     //
     pair<const target_type*, optional<string>>
-    find_target_type (name&, const location&) const;
+    find_target_type (name&,
+                      const location&,
+                      const target_type* = nullptr) const;
 
     // As above but process the potentially out-qualified target name further
     // by completing (relative to this scope) and normalizing the directories
@@ -401,21 +404,22 @@ namespace build2
     //
   public:
     rule_map rules;
+    vector<unique_ptr<adhoc_rule_pattern>> adhoc_rules;
 
     template <typename T>
     void
-    insert_rule (action_id a, const char* hint, const rule& r)
+    insert_rule (action_id a, string hint, const rule& r)
     {
-      rules.insert<T> (a, hint, r);
+      rules.insert<T> (a, move (hint), r);
     }
 
     template <typename T>
     void
     insert_rule (meta_operation_id mid, operation_id oid,
-                 const char* hint,
+                 string hint,
                  const rule& r)
     {
-      rules.insert<T> (mid, oid, hint, r);
+      rules.insert<T> (mid, oid, move (hint), r);
     }
 
     // Operation callbacks.
@@ -579,8 +583,8 @@ namespace build2
     friend LIBBUILD2_SYMEXPORT scope& create_bootstrap_inner (scope&,
                                                               const dir_path&);
 
-    scope (context& c, bool global)
-      : ctx (c), vars (c, global), target_vars (c, global) {}
+    scope (context&, bool global);
+    ~scope ();
 
     // Return true if this root scope can be amalgamated.
     //