Makefile: support ccache for compiling ABC.

Surround $(CC), $(CXX) with double quotes when calling depends.sh, to allow space-delimited compilation tools to be used.

Makefile

@@ -186,15 +186,15 @@ DEP := $(OBJ:.o=.d)
 
 %.d: %.c
 	@echo "$(MSG_PREFIX)\`\` Generating dependency:" $(LOCAL_PATH)/$<
-	$(VERBOSE)$(ABCSRC)/depends.sh $(CC) `dirname $*.c` $(OPTFLAGS) $(INCLUDES) $(CFLAGS) $< > $@
+	$(VERBOSE)$(ABCSRC)/depends.sh "$(CC)" `dirname $*.c` $(OPTFLAGS) $(INCLUDES) $(CFLAGS) $< > $@
 
 %.d: %.cc
 	@echo "$(MSG_PREFIX)\`\` Generating dependency:" $(LOCAL_PATH)/$<
-	$(VERBOSE)$(ABCSRC)/depends.sh $(CXX) `dirname $*.cc` $(OPTFLAGS) $(INCLUDES) $(CXXFLAGS) $< > $@
+	$(VERBOSE)$(ABCSRC)/depends.sh "$(CXX)" `dirname $*.cc` $(OPTFLAGS) $(INCLUDES) $(CXXFLAGS) $< > $@
 
 %.d: %.cpp
 	@echo "$(MSG_PREFIX)\`\` Generating dependency:" $(LOCAL_PATH)/$<
-	$(VERBOSE)$(ABCSRC)/depends.sh $(CXX) `dirname $*.cpp` $(OPTFLAGS) $(INCLUDES) $(CXXFLAGS) $< > $@
+	$(VERBOSE)$(ABCSRC)/depends.sh "$(CXX)" `dirname $*.cpp` $(OPTFLAGS) $(INCLUDES) $(CXXFLAGS) $< > $@
 
 ifndef ABC_MAKE_NO_DEPS
 -include $(DEP)