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, 1 insertions, 1 deletions

diff --git a/tests/dependency/recipe/testscript b/tests/dependency/recipe/testscript
index f43111e..a581724 100644
--- a/tests/dependency/recipe/testscript
+++ b/tests/dependency/recipe/testscript
@@ -406,7 +406,7 @@ alias{x}:
   echo
 }
 EOI
-<stdin>:3:1: error: expected recipe block instead of '{'
+<stdin>:3:1: error: expected recipe block or 'recipe' instead of '{'
 EOE
 
 : duplicate-action-single