Add support for dynamic dependencies in ad hoc Buildscript recipes

Specifically, add the new `depdb dyndep` builtin that can be used to extract
dynamic dependencies from a program run or a file. For example:

obje{hello.o}: cxx{hello}
{{
  s = $path($<[0])

  depdb dyndep $cxx.poptions $cc.poptions --what=header --default-prereq-type=h -- $cxx.path $cxx.poptions $cc.poptions $cxx.mode -M -MG $s

  diag c++ ($<[0])

  o = $path($>)
  $cxx.path $cxx.poptions $cc.poptions $cc.coptions $cxx.coptions $cxx.mode -o $o -c $s
}}

Currently only the `make` dependency format is supported.

1 files changed, 13 insertions, 2 deletions

diff --git a/libbuild2/script/run.hxx b/libbuild2/script/run.hxx
index 477dd88..8bc246c 100644
--- a/libbuild2/script/run.hxx
+++ b/libbuild2/script/run.hxx
@@ -38,11 +38,22 @@ namespace build2
     // Location is the start position of this command line in the script. It
     // can be used in diagnostics.
     //
+    // Optionally, save the command output into the referenced variable. In
+    // this case assume that the expression contains a single pipline.
+    //
     void
-    run (environment&, const command_expr&, size_t index, const location&);
+    run (environment&,
+         const command_expr&,
+         size_t index,
+         const location&,
+         string* output = nullptr);
 
     bool
-    run_if (environment&, const command_expr&, size_t, const location&);
+    run_if (environment&,
+            const command_expr&,
+            size_t index,
+            const location&,
+            string* output = nullptr);
 
     // Perform the registered special file cleanups in the direct order and
     // then the regular cleanups in the reverse order.