Add attract-plus frontend

[jecaro]

Draft PR to try to make attract-plus runs. The PR adds the derivation developed in #16. However, the configuration doesn't build as it requires gbm:

error: Cannot build '/nix/store/fn5frnr3h6s8drl73hnrwxfff8s3d15w-attractplus-3.2.0.drv'.
       Reason: builder failed with exit code 2.
       Output paths:
         /nix/store/s4jbpw5r1a7ga1p3m36sp3xl3gfn1mr6-attractplus-3.2.0
       Last 15 log lines:
       > Running phase: unpackPhase
       > unpacking source archive /nix/store/af3jgmax8pzyfgaw1jhj38n02j45r5i5-source
       > source root is source
       > Running phase: patchPhase
       > Patching Makefile to fix Boost linking...
       > Running phase: updateAutotoolsGnuConfigScriptsPhase
       > Running phase: configurePhase
       > no configure script, doing nothing
       > Running phase: buildPhase
       > build flags: -j24 SHELL=/nix/store/6cankpvayjgidrbx5sj1gms2s13nvfvh-bash-5.3p3/bin/bash prefix=/nix/store/s4jbpw5r1a7ga1p3m36sp3xl3gfn1mr6-attractplus-3.2.0 USE_DRM=1 USE_GLES=1 STATIC=1 USE_SYSTEM_SFML=0
       > Package gbm was not found in the pkg-config search path.
       > Perhaps you should add the directory containing `gbm.pc'
       > to the PKG_CONFIG_PATH environment variable
       > No package 'gbm' found
       > Makefile:419: *** pkg-config couldn't find some libraries, aborting.  Stop.

gbm has been removed from the mesa derivation to save space. We need to figure out if it is really needed to compile attract-plus. Indeed, retroarch and the emulators work fine without it.

• If it is needed, we need to update the mesa overlay to add it back. Something that can be tried is to remove the mesa overlay and try again to build the derivation.
• If not, we need to figure out the compilation flags needed to do without or even patch upstream to make it work

Also, one needs to find out if compiling statically is really needed here. If we can avoid it it would save space.