resolving a kbuild conflict - in-kernel vs module

[Jim Cromie <jcromie@xxxxxxxxxx>]

Ive worked up a patchset to add support for PC-87366 GPIO,
it derives from SCx200 GPIO, which it refactors to pull common
stuff into a new file/module;  nsc_gpio

Until today, Ive built all 3 as modules, all based upon the
SCX200_GPIO config item.

Today I added a 2nd item: PC87366_GPIO, as follows:

obj-$(CONFIG_SCx200_GPIO) += scx200_gpio.o nsc_gpio.o
obj-$(CONFIG_PC87366_GPIO) += pc87366_gpio.o nsc_gpio.o

This seems to work for normal configs; choosing either item above
results in nsc_gpio.ko also being built & installed.

BUT, when I purposely mis-configure, ie build 1 as module, other as 
built-in,
things break: the built module is missing its dependency (which may be 
built into
the kernel properly, but its apparently unavailable to the module)

So the questions:

How do I prevent a mis-configuration like this ?

this would work...
ifneq (obj-$(CONFIG_SCx200_GPIO),n)
   ifneq (obj-$(CONFIG_PC87366_GPIO), n)
      ifneq (obj-$(CONFIG_SCx200_GPIO), obj-$(CONFIG_PC87366_GPIO))
            error ()

but how to do the error ()

Or is this just sweating the details ?

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