configure.ac: don't call GTK_DOC_CHECK twice (fd.o #23943)
configure.ac: don't call GTK_DOC_CHECK twice (fd.o #23943)

freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,noonien-d/wocky,noonien-d/wocky