build: add CONFIG_LINUX

So I can add a tap-linux.c and use CONFIG_LINUX to pull it in
in Makefile
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -1897,6 +1897,10 @@ else
   echo "CONFIG_POSIX=y" >> $config_host_mak
 fi
 
+if test "$linux" = "yes" ; then
+  echo "CONFIG_LINUX=y" >> $config_host_mak
+fi
+
 if test "$darwin" = "yes" ; then
   echo "CONFIG_DARWIN=y" >> $config_host_mak
 fi