Fix: Remove generated .sfo file on clean

This fixes a part of #47.

Makefile.config

@@ -9,6 +9,7 @@ GENERATE_GRF       := sound
 BASE_FILENAME      := opensfx
 SOUND_FILE         := $(BASE_FILENAME).cat
 OBS_FILE           := $(BASE_FILENAME).obs
+SFO_FILE           := $(BASE_FILENAME).sfo
 
 SOUND_FILES        := $(ls src/wav/*.wav)
 LANG_FILES         := $(ls lang/*.lng)

Makefile.in

@@ -9,7 +9,7 @@
 		> $@
 	$(_V) [ -z "$(UNIX2DOS)" ] || $(UNIX2DOS) $(UNIX2DOS_FLAGS) $@
 
-MD5SUM         ?= $(shell command -v md5sum 2>/dev/null)
+MD5SUM         ?= $(shell which md5sum 2>/dev/null)
 
 $(SOUND_FILE): $(SRC_DIR)/$(BASE_FILENAME).sfo $(SOUND_FILES) Makefile Makefile.in Makefile.config
 # replace the place holders for version and name by the respective variables:
@@ -18,7 +18,7 @@ $(SOUND_FILE): $(SRC_DIR)/$(BASE_FILENAME).sfo $(SOUND_FILES) Makefile Makefile.
 	$(_V) $(CATCODEC) $(CATCODEC_FLAGS) $(SRC_DIR)/$@
 	$(_V) cp $(SRC_DIR)/$@ .
 
-ifneq ("$(shell command -v $(MD5SUM) 2>/dev/null)","")
+ifneq ("$(shell which $(MD5SUM) 2>/dev/null)","")
 $(OBS_FILE): $(SOUND_FILE) $(LANG_FILES) Makefile Makefile.in Makefile.config
 	$(_E) "[Generating:] $@"
 	@echo "[metadata]" > $@
@@ -52,3 +52,4 @@ clean::
 	$(_V) -rm -f $(OBS_FILE)
 	$(_V) -rm -f $(SOUND_FILE)
 	$(_V) -rm -f src/$(SOUND_FILE)
+	$(_V) -rm -f src/$(SFO_FILE)