acinclude.m4: Do not set vswitchd_ovs_vswitchd_LDFLAGS

Commit d731058395cb used vswitchd_ovs_vswitchd_LDFLAGS to link with DPDK. This
made automake ignore AM_LDFLAGS while linking the daemon.

This commit explicitly adds AM_LDFLAGS to vswitchd_ovs_vswitchd_LDFLAGS and
moves it to vswitchd/automake.mk

Reported-by: Gurucharan Shetty <gshetty@nicira.com>
Suggested-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>