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, 15 insertions, 7 deletions

diff --git a/libbuild2/adhoc-rule-buildscript.cxx b/libbuild2/adhoc-rule-buildscript.cxx
index c94b50f..c4b9169 100644
--- a/libbuild2/adhoc-rule-buildscript.cxx
+++ b/libbuild2/adhoc-rule-buildscript.cxx
@@ -26,7 +26,6 @@ namespace build2
   recipe_text (context& ctx,
                const scope& s,
                const target* tg,
-               const adhoc_actions& acts,
                string&& t,
                attributes& as)
   {
@@ -61,7 +60,7 @@ namespace build2
     istringstream is (move (t));
     build::script::parser p (ctx);
 
-    script = p.pre_parse (s, tg, acts,
+    script = p.pre_parse (s, tg, actions,
                           is, loc.file, loc.line + 1,
                           move (diag), as.loc);
 
@@ -104,15 +103,14 @@ namespace build2
     os << ind << string (braces, '}');
   }
 
-  optional<action> adhoc_buildscript_rule::
+  bool adhoc_buildscript_rule::
   reverse_fallback (action a, const target_type& tt) const
   {
     // We can provide clean for a file target if we are providing update.
     //
-    if (a == perform_update_id && tt.is_a<file> ())
-      return perform_clean_id;
-
-    return nullopt;
+    return a == perform_clean_id && tt.is_a<file> () &&
+      find (actions.begin (), actions.end (),
+            perform_update_id) != actions.end ();
   }
 
   recipe adhoc_buildscript_rule::
@@ -143,6 +141,11 @@ namespace build2
       return execute_inner;
     }
 
+    // Inject pattern's ad hoc group members, if any.
+    //
+    if (pattern != nullptr)
+      pattern->apply_adhoc_members (a, t, me);
+
     // Derive file names for the target and its ad hoc group members, if any.
     //
     if (a == perform_update_id || a == perform_clean_id)
@@ -165,6 +168,11 @@ namespace build2
     //
     match_prerequisite_members (a, t);
 
+    // Inject pattern's prerequisites, if any.
+    //
+    if (pattern != nullptr)
+      pattern->apply_prerequisites (a, t, me);
+
     // See if we are providing the standard clean as a fallback.
     //
     if (me.fallback)