build: add --enable-lto to enable Link Time Optimization (bgo #741140)

Takes about 3x as long to build with gcc 4.8, but gcc 4.9
is supposed to speed that up considerably.

Name           Before   After   Saved
-------------------------------------
NetworkManager  1734744  1689728   3%
libnm           1263536   808816  36%
nm-iface-helper  931136   906496   3%
libnm-util       441264   437168   1%
libnm-glib       297064   292960   2%

https://bugzilla.gnome.org/show_bug.cgi?id=741140
core: clean up half-done IP operations when re-entering NEED_AUTH state (bgo #741342)

When the device decides it needs re-auth during IP config and returns
to the NEED_AUTH state, make sure we clean up any half-done IP operations
since they will be re-started after auth is completed and the
IP_CONFIG state is re-entered.

https://bugzilla.gnome.org/show_bug.cgi?id=741342
man: allow installing pre-built manual pages if we can't build it

If we have ENABLE_GTK_DOCS, but not BUILD_SETTING_DOCS, nm-settings.5
nm-settings-keyfile.5 and nm-settings-ifcfg-rh.5 would not be installed.

(It happened on RHEL 7 build, due to missing BuildRequires: pygobject3-base)
core: fix warning when releasing slaves on exit (rh #1169936)

NetworkManager[30304]: <info>  (virbr0): bridge port virbr0-nic was detached
NetworkManager[30304]: (devices/nm-device.c:962):nm_device_release_one_slave: runtime check failed: (reason == NM_DEVICE_STATE_REASON_NONE)
NetworkManager[30304]: <info>  (virbr0-nic): released from master virbr0

If the slave is removed, then the master is already cleaned up so NM
doesn't need to do anything.  5dd48f fixed that but forgot to update
the !configure case, causing the warning but no other problems.

Fixes: 5dd48f7527c67b399ac144f594e035216771d61c
core: fix attaching managed slaves to master devices (rh #1141266)

Broken by 25387cd1ffc4b3135a13e9222c0b0e5dea506c80

When an activation request comes in via D-Bus for a slave, the
slave device's priv->master is set in stage1 in master_ready_cb().
Then nm_device_bring_up() is called on the slave, which triggers
link_changed_cb() and device_link_changed().  That then executes
this code:

if (priv->master)
	nm_device_enslave_slave (priv->master, self, NULL);

which enslaves the slave, but due to the NULL will not configure
the slave.

This code was only meant to be run for externally triggered
master/slave changes.
Merge remote-tracking branches 'fd/lr/veth-managed', 'fd/lr/no-udev' and 'fd/lr/ipv6-tokens' into integration