build: Drop use of GNOME_MAINTAINER_MODE_DEFINES
build: Drop use of GNOME_MAINTAINER_MODE_DEFINES

It is deprecated and doesn’t currently do anything useful.

https://wiki.gnome.org/Projects/GnomeCommon/Migration

https://bugzilla.gnome.org/show_bug.cgi?id=776259

pwithnall/totem-pl-parser,pwithnall/totem-pl-parser