datapath: Fix build with kernel header layout recently adopted by Debian.

datapath: Fix build with kernel header layout recently adopted by Debian.

Recent Debian kernel-header packages divide kernel headers into two
directories: the "common" headers that are not architecture-specific,
which go in a directory named like
    /usr/src/kernel-headers-2.6.31-1-common,
and architecture-specific headers in a directory named, e.g.
    /usr/src/kernel-headers-2.6.31-1-686.
OVS needs to look at the ones in the "common" directory as part of its
configuration process, but the build directory provided on --with-l26 is
the architecture-specific directory.  We also need the
architecture-specific directory, since it is the one that we use as part
of the "make", so we can't simply make the user specify the common
directory on --with-l26.  Furthermore, there is no easy-to-see link
between the two directories, except as part of the text in a Makefile,
which is not the easiest language to parse.

This commit attempts to kluge around the problem by using the Debian
directory naming.  If the build directory does not contain the headers,
then we replace the last component of its name by "-common" and check
for the headers there.  This is not ideal, but it does solve the actual
problem at hand.

Tested with Debian's linux-headers-2.6.31-1-686 and with a few older
sets of headers that do not use this scheme.