Problem compiling the client in Kubuntu Edgy

I try to do the ./configure but it fails in the following check:

...
checking if enabling -pie/fPIE support... yes
checking crt_externs.h usability... no
checking crt_externs.h presence... no
checking for crt_externs.h... no
checking for _NSGetEnviron... no
checking for vsnprintf... yes
checking for snprintf... yes
checking for X... configure: error: Can't find X includes. Please check your installation and add the correct paths!

¿What library do I need for that "X" check?

I use Kubuntu Edgy with g++ installed.

Thanks.