@MAKE -C dir1 dir2 -f Makefile1 $(filter-out _all sub-make,$(MAKECMDGOALS)) explain the grammar in this snippet

Sure, here is an explanation of the grammar in this snippet:

- `@MAKE`: This tells Make to run the command `MAKE` silently, without displaying the command itself.
- `-C dir1 dir2`: This tells Make to change to the directory `dir1/dir2` before running the rest of the command.
- `-f Makefile1`: This tells Make to use the file `Makefile1` instead of the default `Makefile` for this command.
- `$(filter-out _all sub-make,$(MAKECMDGOALS))`: This is a Make function that filters out any targets that match `_all` or `sub-make` from the list of goals specified on the command line (`$(MAKECMDGOALS)`). The resulting list of targets is used as the goals for this Make command.

Put together, this command runs Make silently, changes to the directory `dir1/dir2`, uses `Makefile1` as the Makefile, and filters out any targets that match `_all` or `sub-make` from the list of goals specified on the command line. The resulting list of targets is then used as the goals for this Make command.