gnu: slim: Upgrade to 1.3.6; allow the choice of a config file at run time.

* gnu/packages/slim.scm (slim): Upgrade to 1.3.6.  Add patches.  Comment
  out systemd-related stuff from CMakeLists.txt.  Add
  "-DBUILD_SHARED_LIBS=OFF" and "-DCMAKE_SKIP_BUILD_RPATH=ON" to
  #:configure-flags.  Adjust 'home-page' field.
* gnu/packages/patches/slim-config.patch,
  gnu/packages/patches/slim-session.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.