Add ability to specify recipes in separate files

This can now be achieved with the new `recipe` directive:

recipe <language> <file>

Note that similar to the use of if-else and switch directives with recipes,
this directive requires explicit % recipe header. For example, instead of:

file{foo.output}:
{{
   echo 'hello' >$path($>)
}}

We can now write:

file{foo.output}:
%
recipe buildscript hello.buildscript

With hello.buildscript containing:

echo 'hello' >$path($>)

Similarly, for C++ recipes (this time for a pattern), instead of:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
{{ c++ 1 --

--

...

}}

We can now write:

[rule_name=hello] file{~'/(.+)\.output/'}:
% update clean
recipe c++ hello.cxx

With hello.cxx containing:

// c++ 1 --

--

...

Relative <file> paths are resolved using the buildfile directory that contains
the `recipe` directive as a base.

Note also that this mechanism can be used in exported buildfiles with recipe
files placed into build/export/ together with buildfiles.

1 files changed, 49 insertions, 0 deletions

diff --git a/libbuild2/target.cxx b/libbuild2/target.cxx
index a70830e..2a134a4 100644
--- a/libbuild2/target.cxx
+++ b/libbuild2/target.cxx
@@ -1775,6 +1775,55 @@ namespace build2
     target_type::flag::none
   };
 
+  static const char*
+  buildscript_target_extension (const target_key& tk, const scope*)
+  {
+    // If the name is special 'buildscript', then there is no extension,
+    // otherwise it is .buildscript.
+    //
+    return *tk.name == "buildscript" ? "" : "buildscript";
+  }
+
+  static bool
+  buildscript_target_pattern (const target_type&,
+                             const scope&,
+                             string& v,
+                             optional<string>& e,
+                             const location& l,
+                             bool r)
+  {
+    if (r)
+    {
+      assert (e);
+      e = nullopt;
+    }
+    else
+    {
+      e = target::split_name (v, l);
+
+      if (!e && v != "buildscript")
+      {
+        e = "buildscript";
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  const target_type buildscript::static_type
+  {
+    "buildscript",
+    &file::static_type,
+    &target_factory<buildscript>,
+    &buildscript_target_extension,
+    nullptr,  /* default_extension */
+    &buildscript_target_pattern,
+    nullptr,
+    &file_search,
+    target_type::flag::none
+  };
+
   const target_type doc::static_type
   {
     "doc",