Fix syzygy dependencies issue

fixes https://github.com/official-stockfish/Stockfish/issues/2660

The problem was caused by .depend being created with a rule for tbprobe.o not for syzygy/tbprobe.o.
This patch keeps an explicit list of sources (SRCS), generates OBJS,
and compiles all object files to the src/ directory, consistent with .depend.
VPATH is used to search the syzygy directory as needed.

joint work with @gvreuls

closes https://github.com/official-stockfish/Stockfish/pull/2664

No functional change

src/Makefile

@@ -35,10 +35,14 @@ BINDIR = $(PREFIX)/bin
 ### Built-in benchmark for pgo-builds
 PGOBENCH = ./$(EXE) bench
 
-### Object files
-OBJS = benchmark.o bitbase.o bitboard.o endgame.o evaluate.o main.o \
-	material.o misc.o movegen.o movepick.o pawns.o position.o psqt.o \
-	search.o thread.o timeman.o tt.o uci.o ucioption.o tune.o syzygy/tbprobe.o
+### Source and object files
+SRCS = benchmark.cpp bitbase.cpp bitboard.cpp endgame.cpp evaluate.cpp main.cpp \
+	material.cpp misc.cpp movegen.cpp movepick.cpp pawns.cpp position.cpp psqt.cpp \
+	search.cpp thread.cpp timeman.cpp tt.cpp uci.cpp ucioption.cpp tune.cpp syzygy/tbprobe.cpp
+
+OBJS = $(notdir $(SRCS:.cpp=.o))
+
+VPATH = syzygy
 
 ### Establish the operating system name
 KERNEL = $(shell uname -s)
@@ -450,7 +454,7 @@ objclean:
 # clean auxiliary profiling files
 profileclean:
 	@rm -rf profdir
-	@rm -f bench.txt *.gcda ./syzygy/*.gcda *.gcno ./syzygy/*.gcno
+	@rm -f bench.txt *.gcda *.gcno
 	@rm -f stockfish.profdata *.profraw
 
 default:
@@ -536,7 +540,7 @@ icc-profile-use:
 	all
 
 .depend:
-	-@$(CXX) $(DEPENDFLAGS) -MM $(OBJS:.o=.cpp) > $@ 2> /dev/null
+	-@$(CXX) $(DEPENDFLAGS) -MM $(SRCS) > $@ 2> /dev/null
 
 -include .depend