coupler/Makefile: use implicit rules

coupler/Makefile

 CC=gcc
-CFLAGS= -I $(OPEN62541_SOURCE_HOME) 
+CFLAGS= -I $(OPEN62541_SOURCE_HOME) -std=c99 $(C_COMPILER_EXTRA_FLAGS)
 LDFLAGS= -L $(OPEN62541_HOME)/lib
-EXTRA_FLAGS=$(C_COMPILER_EXTRA_FLAGS)
 OUT_DIR= bin
 
 server: server.c
-	$(CC) -o $@ $^ $(CFLAGS) $(LDFLAGS) -std=c99 $(EXTRA_FLAGS)
 
 install:
 	mkdir -p $(OUT_DIR)