build: Add minimum and maximum API version checks for dependencies
build: Remove unused macros from configure.ac

Their functionality was either outdated (GNOME_MAINTAINER_MODE_DEFINES),
or unused (GNOME_DEBUG_CHECK).
build: Switch from GNOME_CODE_COVERAGE to AX_CODE_COVERAGE

AX_CODE_COVERAGE comes from the autoconf-archive, and is an upstreamed
version of the original GNOME_CODE_COVERAGE macro.

https://savannah.gnu.org/patch/?8451

By switching to it, we reduce our dependency on gnome-common. The master
plan is for gnome-common to simply cease to exist.

See: https://bugzilla.gnome.org/show_bug.cgi?id=729405
build: Copy GNOME_COMPILE_WARNINGS in tree

The master plan is for GNOME_COMPILE_WARNINGS to be upstreamed to
autoconf-archive, and then removed from gnome-common. That hasn’t
happened yet, but in the mean time we can reduce our dependency on
gnome-common by copying it in tree.

See: https://bugzilla.gnome.org/show_bug.cgi?id=729407
build: Eliminate dependency on gnome-common

Remove our usage of gnome-autogen.sh by inlining it and deleting
irrelevant stuff. This removes libgdata’s dependency on gnome-common,
which will make it a little easier to build.
build: Directly encode maximum GLib version

If we use the GLIB_VERSION_2_38 macro, compilation will fail with older
versions of GLib which do not have that macro defined. Use the
underlying G_ENCODE_VERSION() macro instead.
freebase: Remove a redundant comparison with zero

Unsigned values can never be less than zero.
configure: Use stronger quoting for GLIB_MAX_ALLOWED

Because the shell is trying to interpret '(', use single quotes
around the value.

Fixes the build in Continuous.
build: Remove .travis.yml

Having spent the best part of a day iterating (in a branch) to get the
script working, I have decided it is just not worth the time.
gnome-continuous is much more suited to libgdata, and is a lot easier to
use.

Remove the .travis.yml file so nobody is tempted to believe it is
supported or works. If they ever upgrade from Ubuntu 12.04 and improve
the iteration times for setting up .travis.yml, it might be worth
another try.
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