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.
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 <email@example.com> 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
local:/usr/src/rdesktop-1.8.3$ [color=blue]automake --version[/color] 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 <firstname.lastname@example.org> and Alexandre Duret-Lutz <email@example.com>.
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$ [color=blue]cp /usr/share/automake-1.14/config.guess .[/color] 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 ....