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