compiler-flags: allow suppressing warnings

Some projects want to see no warnings at all, for example, projects that
are written in Vala. But GNOME_COMPILE_WARNINGS([no]) is a no-op that
does nothing to turn off the default compiler warnings. Presumably if
you ask for no compiler warnings, you want no compiler warnings, so
suppress them in this case instead.

https://bugzilla.gnome.org/show_bug.cgi?id=734973
build: Remove a non-existent file from EXTRA_DIST

https://bugzilla.gnome.org/show_bug.cgi?id=735418
macros2: Deprecate GNOME_DEBUG_CHECK

The autoconf-archive AX_DEBUG_CHECK macro should be used instead, or the
macro contents should just be substituted into configure.ac directly,
since it’s so simple.

https://bugzilla.gnome.org/show_bug.cgi?id=729403
macros2: Deprecate GNOME_MAINTAINER_MODE_DEFINES

The modules which it deprecates are all ancient and unused. Many of the
modules which use GNOME_MAINTAINER_MODE_DEFINES don’t make use of the
DISABLE_DEPRECATED variable it generates anyway.

If a module really wants maintainer mode, they should use
AM_MAINTAINER_MODE. Otherwise, they should just eliminate this call
entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=735417
macros2: Deprecate GNOME_CODE_COVERAGE

The autoconf-archive AX_CODE_COVERAGE macro should be used instead.

https://bugzilla.gnome.org/show_bug.cgi?id=729405
macros2: Add support for APPSTREAM_XML macro

And request that appstream-xml.m4 get installed if missing.

https://bugzilla.gnome.org/show_bug.cgi?id=736497
macros2: Update links to macros in the autoconf-archive

Now that the macros have been committed upstream, we can link to their
manual pages rather than the bug reports.
version 3.14.0
macros2: Remove GNOME_COMMON_INIT

It’s been deprecated for a while, and apparently no modules (which I
have checked out, at least) still use it.

https://bugzilla.gnome.org/show_bug.cgi?id=735416
doc-build: Remove doc-build infrastructure

This pertained to the old GNOME documentation infrastructure, used
before Mallard existed. This isn’t used by any modules we care about
any more. Most modules which set USE_COMMON_DOC_BUILD do so erroneously.

https://bugzilla.gnome.org/show_bug.cgi?id=729931
macros2: Remove support for gnome-doc-utils

Remove support for checking for the GNOME_DOC_INIT macro in configure.ac
and calling gnome-doc-prepare if found. gnome-doc-utils has not been
used in any modules we care about for a long time.

https://bugzilla.gnome.org/show_bug.cgi?id=729931