CONFIG_MACH_MP900C is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

Nec Mobilepro 900/c

modulename: mp900.ko

configname: CONFIG_MACH_MP900C

Linux Kernel Configuration
└─>Intel PXA2xx/PXA3xx Implementations
└─>Nec Mobilepro 900/c
In linux kernel since version 2.6.28 (release Date: 2008-12-24)  
source code:
arch/arm/mach-pxa/mp900.c
selects
CONFIG_PXA25x

depends
CONFIG_ARCH_PXA |
| CONFIG_ATAGS or CONFIG_UNUSED_BOARD_FILES