respect the machine file binary overrides, even if it doesn't exist

If someone specifies a binary in a machine file, but the resulting
prog.found() is false because it doesn't actually exist on disk, then
the user was probably trying to disable finding that program. But
find_program() currently doesn't distinguish between a machine file
lookup returning a not-found program, and returning a dummy program
because there's no entry at all.

Explicitly check for a dummy program, rather than checking if the
program was found, before deciding whether to discard the lookup results
and continue trying other program lookup methods.

mesonbuild/interpreter/interpreter.py

@@ -1484,7 +1484,9 @@ class Interpreter(InterpreterBase, HoldableObject):
             if not isinstance(p, str):
                 raise InterpreterException('Executable name must be a string')
             prog = ExternalProgram.from_bin_list(self.environment, for_machine, p)
-            if prog.found():
+            # if the machine file specified something, it may be a regular
+            # not-found program but we still want to return that
+            if not isinstance(prog, NonExistingExternalProgram):
                 return prog
         return None