Add $(OBJDIR) as prerequisite to $(LIBNAMESO)

The directory must exist before running this rule.  When running `make` with
several parallel jobs it can happen that $(LIBNAMESO) rule is run before
creating the directory.

Makefile

@@ -127,12 +127,12 @@ endif
 OBJS = $(addprefix $(OBJDIR), $(OBJ))
 DEPS = $(wildcard src/*.h) Makefile include/darknet.h
 
-all: obj backup results setchmod $(EXEC) $(LIBNAMESO) $(APPNAMESO)
+all: $(OBJDIR) backup results setchmod $(EXEC) $(LIBNAMESO) $(APPNAMESO)
 
 ifeq ($(LIBSO), 1)
 CFLAGS+= -fPIC
 
-$(LIBNAMESO): $(OBJS) include/yolo_v2_class.hpp src/yolo_v2_class.cpp
+$(LIBNAMESO): $(OBJDIR) $(OBJS) include/yolo_v2_class.hpp src/yolo_v2_class.cpp
 	$(CPP) -shared -std=c++11 -fvisibility=hidden -DLIB_EXPORTS $(COMMON) $(CFLAGS) $(OBJS) src/yolo_v2_class.cpp -o $@ $(LDFLAGS)
 
 $(APPNAMESO): $(LIBNAMESO) include/yolo_v2_class.hpp src/yolo_console_dll.cpp
@@ -151,8 +151,8 @@ $(OBJDIR)%.o: %.cpp $(DEPS)
 $(OBJDIR)%.o: %.cu $(DEPS)
 	$(NVCC) $(ARCH) $(COMMON) --compiler-options "$(CFLAGS)" -c $< -o $@
 
-obj:
-	mkdir -p obj
+$(OBJDIR):
+	mkdir -p $(OBJDIR)
 backup:
 	mkdir -p backup
 results: