Worked OK with 2.5.64:
gcc -Wp,-MD,drivers/char/.genrtc.o.d -D__KERNEL__ -Iinclude -Wall -Wstrict-prototypes
-Wno-trigraphs -O2 -fno-strict-aliasing -fno-common -pipe -mpreferred-stack-boundary=2
-march=athlon -Iinclude/asm-i386/mach-default -fomit-frame-pointer -nostdinc -iwithprefix
include -DKBUILD_BASENAME=genrtc -DKBUILD_MODNAME=genrtc -c -o drivers/char/genrtc.o
drivers/char/genrtc.c
drivers/char/genrtc.c:100: warning: static declaration for `gen_rtc_interrupt' follows non-static
drivers/char/genrtc.c: In function `gen_rtc_timer':
drivers/char/genrtc.c:135: warning: comparison of distinct pointer types lacks a cast
drivers/char/genrtc.c: In function `gen_rtc_proc_output':
drivers/char/genrtc.c:453: void value not ignored as it ought to be
drivers/char/genrtc.c:498: `RTC_BATT_BAD' undeclared (first use in this function)
drivers/char/genrtc.c:498: (Each undeclared identifier is reported only once
drivers/char/genrtc.c:498: for each function it appears in.)
make[2]: *** [drivers/char/genrtc.o] Error 1
make[1]: *** [drivers/char] Error 2
make: *** [drivers] Error 2