Add pkg-config support and include compat/linux* in packaging

Add pkg-config support and include compat/linux* in packaging

Different kernel header versions has been a continuing source of pain.
Though I don't prefer this approach, it seems the most reliable way to
get a build (for the time being) is to bundle the linux/bpf*.h in a
compat directory in the target install location.

To get visibility into that compat directory from another library, one
will need to rely on pkg-config --cflags argument to pull in that compat
directory.

Future work will be to avoid bundling the compat directory if the
version of linux-libc-dev is high enough, or if the presence of the
result of 'make headers_install' from the running kernel can be
detected.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>