locator: Only start IP search if network is available

We were always starting an IP based geolocation, whether or
not the network was available. We should only start it
if the network is available.

https://bugs.freedesktop.org/show_bug.cgi?id=69048
locator: Check for cancelled error correctly

https://bugs.freedesktop.org/show_bug.cgi?id=69049
locator: Don't needlessly hold a reference to ourselves

We don't need to hold a reference to the locator, as this
stops it from getting unreferenced from outside the object,
and thus the async operations from getting cancelled.

https://bugs.freedesktop.org/show_bug.cgi?id=69049
Add missing static keywords to internal functions

https://bugs.freedesktop.org/show_bug.cgi?id=69102
Enable compiler warnings by default

Drop the --enable-strict-cc configure option and replace it with
--enable-compile-warnings from gnome-common. This is enabled by default
to make sure warnings show up during development.

https://bugs.freedesktop.org/show_bug.cgi?id=69102
Avoid the dependency on gnome-common

Ship gnome-compiler-flags.m4 in m4/ so that we wouldn't need the whole
gnome-common during build time.

https://bugs.freedesktop.org/show_bug.cgi?id=69102
Minor pkgconfig file cleanup

Drop the Requires: as we don't ship a shared library, and add
datarootdir to stop autotools from complaining.

https://bugs.freedesktop.org/show_bug.cgi?id=69102
Update .gitignore

https://bugs.freedesktop.org/show_bug.cgi?id=69102
Fix distcheck

Dist org.freedesktop.GeoClue2.service.in so that it gets included in
tarballs.

https://bugs.freedesktop.org/show_bug.cgi?id=69102
locator: Really take distance threshold into account

This fixes several issues that led to ignoring the DistanceThreshold
value most of the time:

 - we were ignoring the threshold value when the description from the
   IP geolocation service changed;

 - double equality comparison was done with == and not taking epsilon
   into account;

 - location threshold was interpreted in km, whereas most application
   authors expect it to be in meters.

https://bugs.freedesktop.org/show_bug.cgi?id=69105
interface: Add a comment for DistanceThreshold

Clarify that the value is in meters.

https://bugs.freedesktop.org/show_bug.cgi?id=69105