lix: only use LTO with GCC

Enabling LTO causes test failures when compiling for x86_64-darwin (at
least under Rosetta 2). The Nix package only enables LTO when using GCC,
so we'll copy that for now. I don't feel safe leaving it on for
aarch64-darwin unless we can understand why it's failing on
x86_64-darwin in case this is an indication of UB.

Fixes #337036.

pkgs/tools/package-management/lix/common.nix

@@ -190,7 +190,7 @@ stdenv.mkDerivation {
     [
       # Enable LTO, since it improves eval performance a fair amount
       # LTO is disabled on static due to strange linking errors
-      (lib.mesonBool "b_lto" (!stdenv.hostPlatform.isStatic))
+      (lib.mesonBool "b_lto" (!stdenv.hostPlatform.isStatic && stdenv.cc.isGNU))
       (lib.mesonEnable "gc" true)
       (lib.mesonBool "enable-tests" true)
       (lib.mesonBool "enable-docs" enableDocumentation)