For what it's worth, I downloaded rdesktop-1.6.0.tar.gz and tried to do a ./configure, I see:

checking for stdlib.h... yes
checking for string.h... yes
checking for memory.h... yes
checking for strings.h... yes
checking for inttypes.h... yes
checking for stdint.h... yes
checking for unistd.h... yes
checking whether byte ordering is bigendian... no
checking for X... no

ERROR: Could not find X Window System headers/libraries.
Probably you need to install the libx11-dev package.
To specify paths manually, use the options --x-includes and --x-libraries.

knoppix@Microknoppix:/media/sdc1/rdesktop/rdesktop-1.6.0$

I'm running a terminal emulator in x, why doesn't it see it?

-Charlie