build: link each text-font backend separately

build: link each text-font backend separately

We want to modularize the text-font library, so we need each backend as a
different library. This step links each of them into a different static
library so we can now link them either into the binary or into modules.

This step also makes 8x16 always compiled into the text-font backend so we
always have a working implementation even though no modules might be
loaded.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>