docs: make -j4 in case of error;

but error may still exist due to incomplete dependency

README.md

@@ -21,7 +21,7 @@ This system is really user-friendly and you can pick it up in several minutes. R
 - fork this repository in your github account
 
 Then you need to compile the project, just type `make` in the gStore root directory, then all executables will be generated.
-(For faster compiling speed, use `make -j8` instead)
+(For faster compiling speed, use `make -j4` instead)
 
 The first strategy is suggested to get the source code because you can easily acquire the updates of the code by typing `git pull` in the home directory of gStore repository. 
 In addition, you can directly check the version of the code by typing `git log` to see the commit logs.

makefile

@@ -41,7 +41,7 @@ CC = g++
 #when developing, not use -O because it will disturb the normal 
 #routine. use it for test and release.
 CFLAGS = -c -Wall -O2 -pthread -std=c++11
-XEFLAG = -O2 -pthread -std=c++11
+EXEFLAG = -O2 -pthread -std=c++11
 #-coverage
 #CFLAGS = -c -Wall -pthread -g -std=c++11 -pg
 #EXEFLAG = -pthread -g -std=c++11 -pg