compat: Fixups for newer kernels

compat: Fixups for newer kernels

A recent patch series added support for ERSPAN but left some problems
remaining for kernel releases from 4.10 to 4.14.  This patch
addresses those problems.

Of note is that the old cisco gre compat layer code is gone for good.

Also, several compat defines in acinclude.m4 were looking for keys
in .c source files - this does not work on distros without source
code.  A more reliable key was already defined so we use that instead.

We have pared support for the Linux kernel releases in .travis.yml
to reflect that 4.15 is no longer in the LTS list.  With this patch
the Out of Tree OVS datapath kernel modules can build on kernels
up to 4.14.47.  Support for kernels up to 4.16.x will be added
later.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>