Makefile: Compile Linux-specific files based on __linux__ macro.

Makefile: Compile Linux-specific files based on __linux__ macro.

We want to conditionally compile several files based on whether we're
building for a Linux host, so we need some Automake conditional for that.
Previously this was based on whether Netlink is available and we're not
on ESX (since ESX has Netlink but isn't Linux), but it's more
straightforward to just test for Linux directly.

CC: Luigi Rizzo <rizzo@iet.unipi.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>