MINOR: build: Allow linking to device-atlas library file

DeviceAtlas might be installed in a location where a user might not have
enough permissions to write json.o and dac.o
diff --git a/Makefile b/Makefile
index f055a08..b4afcfb 100644
--- a/Makefile
+++ b/Makefile
@@ -623,8 +623,12 @@
 DEVICEATLAS_SRC =
 DEVICEATLAS_INC = $(DEVICEATLAS_SRC)
 DEVICEATLAS_LIB = $(DEVICEATLAS_SRC)
+ifeq ($(DEVICEATLAS_SRC),)
+OPTIONS_LDFLAGS += -lda
+else
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/json.o
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/dac.o
+endif
 OPTIONS_OBJS	+= src/da.o
 OPTIONS_CFLAGS += -DUSE_DEVICEATLAS $(if $(DEVICEATLAS_INC),-I$(DEVICEATLAS_INC))
 BUILD_OPTIONS  += $(call ignore_implicit,USE_DEVICEATLAS)