fix configure script to correctly set USE_TARGET_OFFLOAD if offload is enabled

build/configure.sh

@@ -805,7 +805,7 @@ EOF
     fi
     if [ "x$result" = "x0" ]; then
 	echo "yes"
-	OFFLOADFLAGS=" -fcf-protection=check -foffload=nvptx-none=\"-O${CXX_OPT}${CXX_DBG} -fcf-protection=check -fopt-info -lm -latomic -lgomp\""
+	OFFLOADFLAGS=" -DUSE_TARGET_OFFLOAD -fcf-protection=check -foffload=nvptx-none=\"-O${CXX_OPT}${CXX_DBG} -fcf-protection=check -fopt-info -lm -latomic -lgomp\""
 	if [ "x${OMPFLAGS}" = "x" ]; then
 	    OFFLOADFLAGS="${OFFLOADFLAGS} -fopenmp"
 	fi