configure.ac: Clean up the code for unifont detection for easier futu…

…re modifications and more verbose output on failure:

checking for /usr/local/share/fonts/truetype/unifont/unifont.ttf... no
checking for /usr/local/share/fonts/opentype/unifont/unifont.otf... no
checking for /usr/local/share/fonts/truetype/unifont/unifont_upper.ttf... no
checking for /usr/local/share/fonts/opentype/unifont/unifont_upper.otf... no
checking for /usr/local/share/fonts/truetype/unifont/unifont_csur.ttf... no
checking for /usr/local/share/fonts/opentype/unifont/unifont_csur.otf... no
configure: error: Unable to locate unifont.ttf/unifont.otf (needed by X11, SDL1.x and SDL2). Following paths were tested:
${with_unifontdir_ttf}/unifont.ttf => ${datarootdir}/fonts/truetype/unifont/unifont.ttf => ${prefix}/share/fonts/truetype/unifont/unifont.ttf => /usr/local/share/fonts/truetype/unifont/unifont.ttf
${with_unifontdir_otf}/unifont.otf => ${datarootdir}/fonts/opentype/unifont/unifont.otf => ${prefix}/share/fonts/opentype/unifont/unifont.otf => /usr/local/share/fonts/opentype/unifont/unifont.otf
Please use --with-unifontdir-ttf=/dir/ or
           --with-unifontdir-otf=/dir/ or
           --with-unifont-ttf=/dir/file or
           --with-unifont-otf=/dir/file or
           maybe adjust --prefix or --datarootdir