poppler rbpoppler.c: Drop Poppler < 0.12 support
poppler rbpoppler-fonts-iter.c: Drop Poppler < 0.12 support
poppler rbpoppler-annotation-text.c: Drop Poppler < 0.12 support
poppler rbpoppler-font-info.c: Drop poppler < 0.12 support
poppler extconf.rb: require Poppler 0.12 or later