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, 8 insertions, 3 deletions

diff --git a/libbuild2/build/script/script.hxx b/libbuild2/build/script/script.hxx
index e11cb45..9d7567c 100644
--- a/libbuild2/build/script/script.hxx
+++ b/libbuild2/build/script/script.hxx
@@ -29,6 +29,10 @@ namespace build2
       using build2::script::deadline;
       using build2::script::timeout;
 
+      // Forward declarations.
+      //
+      class default_runner;
+
       // Notes:
       //
       // - Once parsed, the script can be executed in multiple threads with
@@ -70,9 +74,10 @@ namespace build2
         // The script's custom dependency change tracking lines (see the
         // script parser for details).
         //
-        bool       depdb_clear;
-        lines_type depdb_preamble;
-        bool       depdb_preamble_temp_dir = false; // True if references $~.
+        bool             depdb_clear;
+        optional<size_t> depdb_dyndep;   // Position of first depdb-dyndep.
+        lines_type       depdb_preamble;
+        bool             depdb_preamble_temp_dir = false; // True if refs $~.
 
         location start_loc;
         location end_loc;