CONFIG_ATH79_MACH_UBNT_XM is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Ubiquiti Networks XM (rev 1.0) board

modulename: mach-ubnt-xm.ko

configname: CONFIG_ATH79_MACH_UBNT_XM

Linux Kernel Configuration
└─>Atheros AR71XX/AR724X/AR913X machine selection
└─>Ubiquiti Networks XM (rev 1.0) board
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Say 'Y' here if you want your kernel to support the
Ubiquiti Networks XM (rev 1.0) board.

source code:
arch/mips/ath79/mach-ubnt-xm.c

depends
CONFIG_ATH79