This commit fixes build issues for frontends that wish to use freetype2
or that do not want to use the built-in bitmap font:
* Having HAVE_FREETYPE without DONT_HAVE_BITMAPFONT makes griffin.c
include both freetype.c and bitmapfont.c resulting in a redefinition
of 'struct font_renderer' (freetype.c:25 and bitmapfont.c:24).
* Having both HAVE_FREETYPE and DONT_HAVE_BITMAPFONT resulted in linkage
problems because griffin.c does not include fonts.c in this case and
thus font_renderer_create_default() implementation is missing. If
fonts.c is included, the linker complains about undefined reference to
bitmap_font_renderer in fonts.c:27.
