$ ./configure 
Checking for environment: Determined build machine: linux.x86, target machine: linux.x86, OK.
Checking for kBuild: found, OK.
Checking for gcc: found version 4.2.3, OK.
Checking for as86: found version 0.16.17, OK.
Checking for bcc: found version 0.16.17, OK.
Checking for iasl: found version 20061109, OK.
Checking for xslt: found, OK.
Checking for pthread: found, OK.
Checking for libxml2: found version 2.6.31, OK.
Checking for libxslt: found version 1.1.22, OK.
Checking for libIDL: found version 0.8.10, OK.
Checking for zlib: found version 1.2.3.3, OK.
Checking for libpng: found version 1.2.15beta5, OK.
Checking for SDL: found version 1.2.12, OK.
Checking for X libraries: 
  Xlibs not found at -L/usr/X11R6/lib -L/usr/X11R6/lib64 -L/usr/local/lib -lXext -lX11 -I/usr/local/include or Xlibs headers not found
  Check the file /home/byteshifter/download/VirtualBox-1.6.2/configure.log for detailed error information.