build: portability fixes

- Invoke Python scripts with `python3` if `python` is not available.

  Despite SciresM preferring Python 2, the scripts used in the build all
  worked in Python 3 already, so this is just a Makefile change.

- Export `MAKE` to work around a GNU bug.

  See the comment for details.

- `rmdir --ignore-fail-on-non-empty` -> `rmdir 2>/dev/null`

  macOS `rmdir` doesn't support `--ignore-fail-on-non-empty`.

  This is a slight downgrade in functionality, because now we ignore all
  errors from `rmdir` rather than just 'Directory not empty'.  I could
  have avoided this with a more complicated fix, but the benefit strikes
  me as not worth the complexity.  Let me know if you disagree.

- Append `$(DEVKITPRO)/tools/bin/` to `$PATH` when invoking
  `build_romfs`.

  This avoids the need to add devkitPro directories to `$PATH` when
  building Atmosphere.

  If you already have `build_romfs` in your PATH, then that will still
  take precedence.  (This seemed like the nicer option, though I don't
  have strong opinions.)

fusee/fusee.mk

@@ -43,6 +43,6 @@ clean:
 	@$(MAKE) --no-print-directory -C $(CURRENT_DIRECTORY)/loader_stub -f $(CURRENT_DIRECTORY)/loader_stub/loader_stub.mk clean
 	@$(MAKE) --no-print-directory -C $(CURRENT_DIRECTORY)/program -f $(CURRENT_DIRECTORY)/program/program.mk clean
 	@rm -fr $(CURRENT_DIRECTORY)/$(ATMOSPHERE_OUT_DIR)
-	@for i in $(CURRENT_DIRECTORY)/$(ATMOSPHERE_OUT_DIR); do [ -d $$i ] && rmdir --ignore-fail-on-non-empty $$i || true; done
+	@for i in $(CURRENT_DIRECTORY)/$(ATMOSPHERE_OUT_DIR); do [ -d $$i ] && rmdir $$i 2>/dev/null || true; done
 
 .PHONY: all clean check_lib check_loader_stub check_program

fusee/program/program.mk

@@ -84,7 +84,7 @@ DEPENDS	:=	$(OFILES:.o=.d)
 #---------------------------------------------------------------------------------
 
 $(OUTPUT).lz4	:	$(OUTPUT).bin
-	@python $(CURRENT_DIRECTORY)/lz4_compress.py $(OUTPUT).bin $(OUTPUT).lz4
+	$(SILENTCMD)$(PYTHON) $(CURRENT_DIRECTORY)/lz4_compress.py $(OUTPUT).bin $(OUTPUT).lz4
 	@echo built ... $(notdir $@)
 
 $(OUTPUT).bin	:	$(OUTPUT).elf