Who provides $host_os variable for xserver's configure.ac script?

Mon Jun 22 16:30:20 PDT 2015

I'm figuring out why Debian xorg-server builds are failing to automatically
enable kdrive's evdev input driver. It seems variable "host_os", which is
referenced several times in configure.ac, is undefined when Xorg is built
in Debian. However, I've found no other occurrence of this variable in any
other file in xserver tree.

Where host_os should be defined?
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