Help on X Includes

[Jean-Louis]

Hi:

I'm trying to compile/install a software but I fail because something is
missing.

When I do a ./configure, I got the message "Can't find X includes. Please check
your installation & add the correct paths".

I also tried the command ./configure --x-includes=/usr/X11R6/include/X11.
No more success.

My question are (I'm using RH5.2):
- What exactly are these "includes"? if they're libraries. when could I find or
download them?
- Is  the path correct?

Thanks very much for your help.

Jean Louis