Most common place to get this error is when you try to compile something in cygwin environment because the configuration script is not able to detect properly the system type due to missing important information in config.guess. Therefore, in order to make it work, you need to replace the file with the one available in your system, providing you have installed already automake package.

So, without further writing, this is what you have to do in order to correct the error.

The error:

local:/usr/src/rdesktop-1.8.3$ ./configure
checking build system type... ./config.guess: unable to guess system type

This script, last modified 2003-06-17, has failed to recognize
the operating system you are using. It is advised that you
download the most up to date version of the config scripts from

    ftp://ftp.gnu.org/pub/gnu/config/

If the version you run (./config.guess) is already up to date, please
send the following data and any information you think might be
pertinent to <config-patches@gnu.org> in order to provide the needed
information to handle your system.

config.guess timestamp = 2003-06-17

uname -m = x86_64
uname -r = 1.7.35(0.287/5/3)
uname -s = CYGWIN_NT-6.3
uname -v = 2015-03-04 12:09

/usr/bin/uname -p = unknown
/bin/uname -X     =

hostinfo               =
/bin/universe          =
/usr/bin/arch -k       =
/bin/arch              = x86_64
/usr/bin/oslevel       =
/usr/convex/getsysinfo =

UNAME_MACHINE = x86_64
UNAME_RELEASE = 1.7.35(0.287/5/3)
UNAME_SYSTEM  = CYGWIN_NT-6.3
UNAME_VERSION = 2015-03-04 12:09
configure: error: cannot guess build type; you must specify one

The correction:

local:/usr/src/rdesktop-1.8.3$ automake --version
automake (GNU automake) 1.14.1
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv2+: GNU GPL version 2 or later <http://gnu.org/licenses/gpl-2.0.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Tom Tromey <tromey@redhat.com>
       and Alexandre Duret-Lutz <adl@gnu.org>.

local:/usr/src/rdesktop-1.8.3$ ls -la /usr/share/automake-1.14/config.guess
-rwxr-xr-x 1 USER221 Domain Users 42K Mar 26 15:18 /usr/share/automake-1.14/config.guess*
local:/usr/src/rdesktop-1.8.3$ cp /usr/share/automake-1.14/config.guess .
cp: overwrite ‘./config.guess’? y
local:/usr/src/rdesktop-1.8.3$

Now the configure is successful!

local:/usr/src/rdesktop-1.8.3$ ./configure
checking build system type... x86_64-unknown-cygwin
checking host system type... x86_64-unknown-cygwin
checking for gcc... gcc
....