configure: Fix check for GNU make $(if) extension.

As it turns out, the argument to AC_CONFIG_COMMANDS_PRE gets copied into
config.status whether or not it gets run by the shell at "configure" time,
defeating my attempt to support non-GNU make here.

Reported-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>