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Their functionality was either outdated (GNOME_MAINTAINER_MODE_DEFINES),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: Update .travis.yml