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, 17 insertions, 6 deletions

diff --git a/libbuild2/adhoc-rule-cxx.hxx b/libbuild2/adhoc-rule-cxx.hxx
index 7b83607..8254906 100644
--- a/libbuild2/adhoc-rule-cxx.hxx
+++ b/libbuild2/adhoc-rule-cxx.hxx
@@ -25,6 +25,9 @@ namespace build2
     // in its base.
   };
 
+  // Note that when used as part of a pattern, the implementation cannot use
+  // the match_extra::buffer nor the target auxilary data storage.
+  //
   class LIBBUILD2_SYMEXPORT cxx_rule_v1: public cxx_rule
   {
   public:
@@ -33,11 +36,19 @@ namespace build2
     // cannot be injected as a real prerequisite since it's from a different
     // build context).
     //
-    const location     recipe_loc;   // Buildfile location of the recipe.
-    const target_state recipe_state; // State of recipe library target.
+    // If pattern is not NULL then this recipe belongs to an ad hoc pattern
+    // rule and apply() may need to call the pattern's apply_*() functions if
+    // the pattern has any ad hoc group member substitutions or prerequisite
+    // substitutions/non-patterns, respectively.
+    //
+    const location     recipe_loc;     // Buildfile location of the recipe.
+    const target_state recipe_state;   // State of recipe library target.
+    const adhoc_rule_pattern* pattern; // Ad hoc pattern rule of recipe.
 
-    cxx_rule_v1 (const location& l, target_state s)
-      : recipe_loc (l), recipe_state (s) {}
+    cxx_rule_v1 (const location& l,
+                 target_state s,
+                 const adhoc_rule_pattern* p)
+      : recipe_loc (l), recipe_state (s), pattern (p) {}
 
     // Return true by default.
     //
@@ -56,12 +67,12 @@ namespace build2
     virtual recipe
     apply (action, target&, match_extra&) const override;
 
-    adhoc_cxx_rule (const location&, size_t,
+    adhoc_cxx_rule (string, const location&, size_t,
                     uint64_t ver,
                     optional<string> sep);
 
     virtual bool
-    recipe_text (context&, const scope&, const target*, const adhoc_actions&,
+    recipe_text (context&, const scope&, const target*,
                  string&&, attributes&) override;
 
     virtual