Unify makefiles

[keithbowes]

When doing some experimenting with setting up a translation framework, I noticed that the non-ASCII characters in translations weren't showing correctly, so I went ahead and tried the Cairo version, in which not only did the non-ASCII characters show correctly but also the font was much better. Anyway, I noticed that the Makefile for the Cairo build hadn't been updated in years and had some legacy cruft, so I decided to unify the Makefiles: renaming Makefile to Makefile.cairo, removing everything but what's specifically needed for Cairo builds, and making minor changes in Makefile.libretro and Makefile.common. Now, Makefile.libretro still makes a non-Cairo build by default, but if you want a Cairo build, you just need to set WANT_CAIRO.