Xcode: fix running commands that lie in the build root dir.

mesonbuild/backend/backends.py

@@ -1200,7 +1200,7 @@ class Backend:
                 i = os.path.join(self.get_custom_target_output_dir(i), tmp)
             elif isinstance(i, mesonlib.File):
                 i = i.rel_to_builddir(self.build_to_src)
-                if target.absolute_paths:
+                if target.absolute_paths or absolute_outputs:
                     i = os.path.join(self.environment.get_build_dir(), i)
             # FIXME: str types are blindly added ignoring 'target.absolute_paths'
             # because we can't know if they refer to a file or just a string

mesonbuild/backend/xcodebackend.py

@@ -1002,9 +1002,9 @@ class XCodeBackend(backends.Backend):
         for tname, t in self.custom_targets.items():
             if not isinstance(t, build.CustomTarget):
                 continue
-            (srcs, ofilenames, cmd) = self.eval_custom_target_command(t)
+            (srcs, ofilenames, cmd) = self.eval_custom_target_command(t, absolute_outputs=True)
             fixed_cmd, _ = self.as_meson_exe_cmdline(t.name,
-                                                     t.command[0],
+                                                     cmd[0],
                                                      cmd[1:],
                                                      #workdir=None,
                                                      env=t.env)