[ns] ns-2.28: cannot find -lX11

[Petra]

Hello,

I am trying to install ns-2.28 on Red Hat Enterprise Linux ES release 4.

First, when I run ./configure in otcl-1.9, it gives me this error:

checking for X11 header files
checking for X11 library archive
*checking for XOpenDisplay in -lX11... no
can't find X library*

so I add the X11 library directory and run
./configure --x-libraries=/usr/X11R6/lib/ --x-includes=/usr/X11R6/include/

it passes the checking with a warning:

checking for X11 header files
checking for X11 library archive
checking for libXext.a
*warning: compiling without -lXext*


Then I make,
it says

gcc -o otclsh  -g -O2 -I. -I/srl/ns-allinone-2.28/include
-I/srl/ns-allinone-2.28/include -I/usr/X11R6/include/ -I/include
otclAppInit.c \
-L. -lotcl -L/srl/ns-allinone-2.28/lib -ltk8.4 -L/srl/ns-allinone-2.28/lib
-ltcl8.4 -L/usr/X11R6/lib/  -lX11  -lnsl -ldl -lm
*/usr/bin/ld: cannot find -lX11*
collect2: ld returned 1 exit status
make: *** [otclsh] Error 1

Is this because I do not have libXext?

Or anything else?

Thank you in advance for your help.

Best,
Petra