BUILD: make DeviceAtlas easier to build by defaulting to DEVICEATLAS_SRC

Since both DEVICEATLAS_INC and DEVICEATLAS_LIB are set to the same path
when building from sources, simply allow DEVICEATLAS_SRC to be set alone
to simplify the build procedure.
diff --git a/Makefile b/Makefile
index 219c076..10de1de 100644
--- a/Makefile
+++ b/Makefile
@@ -603,8 +603,11 @@
 endif
 
 ifneq ($(USE_DEVICEATLAS),)
-DEVICEATLAS_INC =
-DEVICEATLAS_LIB =
+# Use DEVICEATLAS_SRC and possibly DEVICEATLAS_INC and DEVICEATLAS_LIB to force path
+# to 51degrees headers and libraries if needed.
+DEVICEATLAS_SRC =
+DEVICEATLAS_INC = $(DEVICEATLAS_SRC)
+DEVICEATLAS_LIB = $(DEVICEATLAS_SRC)
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/json.o
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/dac.o
 OPTIONS_OBJS	+= src/da.o