Qusetion during Installation of Xsim

iicic

Still, i got the  message:

./configure
checking for g++... g++
checking for C++ compiler default output... a.out
checking whether the C++ compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C++ compiler... yes
checking whether g++ accepts -g... yes
checking for gcc... gcc
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking how to run the C preprocessor... gcc -E
checking for ranlib... ranlib
checking for ld... ld
checking for X... no
configure: error: X Window System not found
Why??????????---------------------------------:help