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, 16 insertions, 0 deletions

diff --git a/libbuild2/target.hxx b/libbuild2/target.hxx
index 41bf095..b97d562 100644
--- a/libbuild2/target.hxx
+++ b/libbuild2/target.hxx
@@ -2501,6 +2501,22 @@ namespace build2
     static const target_type static_type;
   };
 
+  // This target type is primarily used for files mentioned in the `recipe`
+  // directive.
+  //
+  class LIBBUILD2_SYMEXPORT buildscript: public file
+  {
+  public:
+    buildscript (context& c, dir_path d, dir_path o, string n)
+      : file (c, move (d), move (o), move (n))
+    {
+      dynamic_type = &static_type;
+    }
+
+  public:
+    static const target_type static_type;
+  };
+
   // Common documentation file target.
   //
   class LIBBUILD2_SYMEXPORT doc: public file