llama-cpp: assert that only one of the *Support arguments is true

When experimenting with llama-cpp I experienced a mysterious build
error that in the end I tracked down to me setting openclSupport and
openblasSupport to true.  To prevent others from this mistake add an
assert that complains if more than one of the arguments openclSupport,
openblasSupport and rocmSupport is set to true.

pkgs/by-name/ll/llama-cpp/package.nix

@@ -22,6 +22,10 @@
 , pkg-config
 }:
 
+assert lib.assertMsg
+  (lib.count lib.id [openclSupport openblasSupport rocmSupport] == 1)
+  "llama-cpp: exactly one  of openclSupport, openblasSupport and rocmSupport should be enabled";
+
 let
   cudatoolkit_joined = symlinkJoin {
     name = "${cudaPackages.cudatoolkit.name}-merged";