kernels with 'options I586_CPU.'
The declaration for pentium_mhz is hidden inside an #ifdef I586_CPU,
but machdep.c refers to it whether I586_CPU is defined or not. This
temporary hack puts the offending code inside an #ifdef I586_CPU as
well so that a kernel without it will successfully compile.
I must emphasize the word 'temporary:' somebody needs to seriously
beat on the identifycpu() function with an #ifdef stick so that
I386_CPU, I486_CPU and I586_CPU will do the right things.