ath9k: fix build of old gmp by disabling compiler warnings (#408390)

Author: vcunat

pkgs/os-specific/linux/firmware/ath9k/default.nix

@@ -47,7 +47,12 @@ stdenv.mkDerivation (finalAttrs: {
     perl
   ];
 
-  env.NIX_CFLAGS_COMPILE = "-w"; # old libiberty emits fatal warnings
+  env.NIX_CFLAGS_COMPILE =
+    # old libiberty emits fatal warnings
+    "-w"
+    # old gmp fails to compile with newer gcc
+    # FIXME remove when the normal version has moved on
+    + lib.optionalString (!enableUnstable) " -fpermissive";
 
   dontUseCmakeConfigure = true;
   enableParallelBuilding = true;