libX11.so.6

[Pivi, Mauro Torino Francesco]

I have just installed PETSC on a x86_64-linux machine.

I am at the level of running the first exercises and after "make" and running the ex2.c file in parallel I get the message

"./ex2: error while loading shared libraries: libX11.so.6: cannot open shared object file: No such file or directory"

WHAT SHOULD I DO??
I spent few days with this. Please, help.

PS: I used the link found the file libX11.so.6 in the directory /usr/X11R6/lib. Thus I run configure.py again with the option
--with-x-include=/usr/X11R6/lib 
But NO IMPROOVEMENT.

Thank you!
Mauro