BUILD: make 51D easier to build by defaulting to 51DEGREES_SRC

Till now 3 paths were needed, 51DEGREES_SRC, 51DEGREES_INC, and
51DEGREES_LIB.  Let's make the last two default to 51DEGREES_SRC since
it's the same location, and fix the doc to reflect this (all three were
documented but inconsistently).
diff --git a/Makefile b/Makefile
index c353b03..219c076 100644
--- a/Makefile
+++ b/Makefile
@@ -613,9 +613,11 @@
 endif
 
 ifneq ($(USE_51DEGREES),)
-# Use 51DEGREES_INC and 51DEGREES_LIB to force path to 51degrees headers and libraries if needed.
-51DEGREES_INC =
-51DEGREES_LIB =
+# Use 51DEGREES_SRC and possibly 51DEGREES_INC and 51DEGREES_LIB to force path
+# to 51degrees headers and libraries if needed.
+51DEGREES_SRC =
+51DEGREES_INC = $(51DEGREES_SRC)
+51DEGREES_LIB = $(51DEGREES_SRC)
 OPTIONS_CFLAGS  += -DUSE_51DEGREES $(if $(51DEGREES_INC),-I$(51DEGREES_INC))
 BUILD_OPTIONS   += $(call ignore_implicit,USE_51DEGREES)
 OPTIONS_LDFLAGS += $(if $(51DEGREES_LIB),-L$(51DEGREES_LIB)) -lz
diff --git a/README b/README
index c372ce0..3d7df12 100644
--- a/README
+++ b/README
@@ -245,24 +245,28 @@
 need. Theses attributes of the device making a web request can be added to HTTP
 headers as configurable parameters.
 
-In order to enable 51Degrees get the 51Degrees source code
-(https://github.com/51Degreesmobi/51Degrees-C) and then run make with
-USE_51DEGREES and 51DEGREES_INC and 51DEGREES_SRC set. Make sure to replace
-'51D_REPO_PATH' with the path to the 51Degrees repository.
+In order to enable 51Degrees get the 51Degrees source code from the official
+github repository :
 
-51Degrees provide 2 different detection algorithms.
+    git clone https://github.com/51Degreesmobi/51Degrees-C
+
+then run 'make' with USE_51DEGREES and 51DEGREES_SRC set. Both 51DEGREES_INC
+and 51DEGREES_LIB may additionally be used to force specific different paths
+for .o and .h, but will default to 51DEGREES_SRC. Make sure to replace
+'51D_REPO_PATH' with the path to the 51Degrees repository.
 
-1. Pattern - balances main memory usage and CPU.
-2. Trie - a very high performance detection solution which uses more main
-   memory than Pattern.
+51Degrees provide 2 different detection algorithms :
+    1. Pattern - balances main memory usage and CPU.
+    2. Trie - a very high performance detection solution which uses more main
+       memory than Pattern.
 
 To make with 51Degrees Pattern algorithm use the following command line.
 
-    $ make TARGET=linux26 USE_51DEGREES=1 51DEGREES_INC='51D_REPO_PATH'/src/pattern 51DEGREES_LIB='51D_REPO_PATH'/src/pattern
+    $ make TARGET=linux26 USE_51DEGREES=1 51DEGREES_SRC='51D_REPO_PATH'/src/pattern
 
 To use the 51Degrees Trie algorithm use the following command line.
 
-    $ make TARGET=linux26 USE_51DEGREES=1 51DEGREES_INC='51D_REPO_PATH'/src/trie 51DEGREES_LIB='51D_REPO_PATH'/src/trie
+    $ make TARGET=linux26 USE_51DEGREES=1 51DEGREES_SRC='51D_REPO_PATH'/src/trie
 
 A data file containing information about devices, browsers, operating systems
 and their associated signatures is then needed. 51Degrees provide a free