Avoid using AC_RUN_IFELSE to check the kernel major.minor version, as it

Avoid using AC_RUN_IFELSE to check the kernel major.minor version, as it
doesn't work when cross-compiling.

Every kernel Makefile I've looked at (at least for 2.4 and 2.6, including
those for separate kernel build directories) includes the "VERSION = x" and
"PATCHLEVEL = y" lines, so use 'sed' to extract the information from those
lines.