Update Makefile, make openmp as default

update document and README

Makefile

@@ -4,13 +4,14 @@ CC = g++ -pg
 CC = g++
 
 # without OpenMP
-CCFLAGS = -DNO_OPENMP
 
 # with OpenMP
 # in command line: 
 # make openmp=yes
-ifeq ($(openmp),yes)
-CCFLAGS = -fopenmp
+ifeq ($(openmp),no)
+  CCFLAGS = -DNO_OPENMP
+else
+  CCFLAGS = -fopenmp
 endif
 
 # support debugging

README

@@ -1,9 +1,8 @@
 For cd-hit
 
 How to compile?
-  1. Compile without multi-threading support: make
-  2. Compile with multi-threading support: make openmp=yes
-
+  1. Compile with multi-threading support (default): make
+  2. Compile without multi-threading support (if you are on very old systems): make openmp=no
 
 
 For cd-hit-auxtools
@@ -15,7 +14,8 @@ For psi-cd-hit
   please download legacy BLAST (not BLAST+) and install the executables in your $PATH
 
 
-For more information, please visit http://cd-hit.org or please read the cd-hit-users-guide.pdf. 
+For more information, please visit http://cd-hit.org or please read the cdhit-users-guide.pdf. 
+Most up-to-date documents are available at http://weizhongli-lab.org/cd-hit/wiki/doku.php?id=cd-hit_user_guide.
 
 cd-hit was originally hosted at Google Code, some of the old releases are still available from https://code.google.com/p/cdhit/.