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
diff --git a/README b/README
index 3d7df12..ab00764 100644
--- a/README
+++ b/README
@@ -211,7 +211,11 @@
 the API source code from https://deviceatlas.com/deviceatlas-haproxy-module and
 once extracted :
 
-    $ make TARGET=<target> USE_PCRE=1 USE_DEVICEATLAS=1 DEVICEATLAS_INC=<path to the API root folder> DEVICEATLAS_LIB=<path to the API root folder>
+    $ make TARGET=<target> USE_PCRE=1 USE_DEVICEATLAS=1 DEVICEATLAS_SRC=<path to the API root folder>
+
+Optionally DEVICEATLAS_INC and DEVICEATLAS_LIB may be set to override the path
+to the include files and libraries respectively if they're not in the source
+directory.
 
 These are supported DeviceAtlas directives (see doc/configuration.txt) :
   - deviceatlas-json-file <path to the DeviceAtlas JSON data file>.