Extend automatic enabling of Kbuild mechanism back as far as kernel 2.6.10

Extend automatic enabling of Kbuild mechanism back as far as kernel 2.6.10
(which is the first Kbuild enabled kernel supported by Comedi) and also
enable Kbuild mechanism if separate kernel source and build directories
are in use.

It ought to be possible to use the Kbuild mechanism back as far as kernel
2.6.8 by checking for the script/mkmakefile file (that's what I tried first,
actually), but use of the Kbuild mechanism for 2.6.8 and 2.6.9 is currently
incompatible with Comedi's build mechanism (I tried it).