BUILD: MAKEFILE: Introduce DETECT_OBJS variable. Executable depends on it.

Note: No detection objects added currently. It's just an empty variable uptill now.
- These DETECT_OBJS will be seen in action in the new commits
- They contain engine_name/detection.o
- They have MetaEngine code, which has detection features.
- This way, Executable will have linked against the detection.o files
- Detection.cpp files will be individually compilable and not dependent on engine

Makefile

@@ -15,6 +15,9 @@ DEPDIR      := .deps
 MODULES     :=
 MODULE_DIRS :=
 
+# All game detection-related object files for engines
+DETECT_OBJS :=
+
 # Load the make rules generated by configure
 -include config.mk
 

Makefile.common

@@ -86,7 +86,7 @@ endif
 endif
 
 # The build rule for the ScummVM executable
-$(EXECUTABLE): $(OBJS)
+$(EXECUTABLE): $(OBJS) $(DETECT_OBJS)
 	$(QUIET_LINK)$(LD) $(LDFLAGS) $(PRE_OBJS_FLAGS) $+ $(POST_OBJS_FLAGS) $(LIBS) -o $@
 
 ifdef SPLIT_DWARF
@@ -99,7 +99,7 @@ distclean: clean clean-devtools
 
 clean:
 	$(RM_REC) $(DEPDIRS)
-	$(RM) $(OBJS) $(EXECUTABLE)
+	$(RM) $(OBJS) $(DETECT_OBJS) $(EXECUTABLE)
 ifdef SPLIT_DWARF
 	$(RM) $(OBJS:.o=.dwo)
 	$(RM) $(EXECUTABLE).dwp