configure: Try to extract kernel source directory from build Makefile.

configure: Try to extract kernel source directory from build Makefile.

OVS needs to inspect the headers in the kernel source directory at build
time.  Debian keeps moving the source directory relative to the build
directory and doesn't provide an obvious way to find the source directory,
so in the past we've used some name-based heuristics to essentially guess
where it is.

This commit introduces a new heuristic that I hope will be more reliable:
extracting the source directory from the Makefile in the build directory.
In Debian's case, it looks like the Makefile generally contains a line of
the form "MAKEARGS := -C <srcdir> O=<outdir>".  This commit extracts the
source directory from that line.

To avoid regressions this commit retains the older heuristics as fallbacks.

CC: 659685@bugs.debian.org
Reported-by: Thomas Goirand <zigo@debian.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>