* gnu/packages/agda.scm (agda): Update to 2.6.4. Also build with
optimizations and cluster counting support.
* gnu/packages/patches/agda-use-sphinx-5.patch: New patch.
* gnu/local.mk (dist_patch_DATA): Register it.
Change-Id: I98a66678ec401b9201d9fd3a0f606fff9767b4bc