Allow the use of $in and $out in exec args

Lib/gftools/builder/operations/exec.py

@@ -11,3 +11,21 @@ def validate(self):
 
         if "args" not in self.original:
             raise ValueError("No arguments given")
+
+    @property
+    def variables(self):
+        # We would like to be able to use the "$in" and
+        # "$out" template variables inside our "args" string,
+        # but ninja does not perform this second level of
+        # variable expansion. So we do it ourselves.
+        vars = {
+            "exe": self.original["exe"],
+            "args": self.original["args"],
+        }
+        all_input_files = " ".join([source.path for source in self._sources])
+        vars["args"] = vars["args"].replace("$in", all_input_files)
+        if "$out" in vars["args"]:
+            if len(self._targets) != 1:
+                raise ValueError("Multiple outputs, but $out used")
+            vars["args"] = vars["args"].replace("$out", self.first_target.path)
+        return vars