package xdmcp was not found in the pkg-config

Hello everybody,

  I'm  trying  to  compile xserver on my Slackware 9.1, but when I run
  ./configure I get this error, and I don't know how to solve:

  checking for xdmcp ... package xdmcp was not found in the pkg-config
  search  path.  Perhaps  you  should  add  the  directory  containing
  'xdmcp.pc'.

  But look this:

  # whereis pkg-config
  pkg-config:    /usr/bin/pkg-config   /usr/man/man1/pkg-config.1.gz
  /usr/share/man/man1/pkg-config.1.gz
  # find / -name xdmcp.pc

  Someone know how to solve this?.

  Thanks in advance ... and please, sorry my poor english.

-- 
Saludos,
 Matías
 Rosario, Santa Fe, Argentina