Can not use same name twice.

coupler/opc-ua-server/Makefile

 CC=arm-linux-gnueabihf-gcc
 CFLAGS= -I $(OPEN62541_SOURCE_HOME) 
 LDFLAGS= -L $(OPEN62541_HOME)/lib
-OUT_DIR=$(OUT_DIR)
+OUT_DIR= $(BINARY_OUT_DIR)
 
 server: server.c
 	$(CC) -o $@ $^ $(CFLAGS) $(LDFLAGS) -std=c99 $(OPEN62541_SOURCE_HOME)/open62541.c

slapos/software/osie-coupler/software-c.cfg

@@ -48,7 +48,7 @@ path = ${osie-repository:location}/coupler/opc-ua-server/
 environment =
   OPEN62541_HOME = ${open62541:location}
   OPEN62541_SOURCE_HOME = ${open62541-source:location}
-  OUT_DIR = ${buildout:directory}/bin
+  BINARY_OUT_DIR = ${buildout:directory}/bin
 # actually configure-command is useless here but needed by convention
 configure-command = 
   ${cmake:location}/bin/cmake