spidermonkey_68: fix cross

The use of unspliced llvmPackages in configureFlags was causing an
eval failure when cross-compiling, but since later versions of
Firefox/Spidermonkey can correctly detect clang and libclang without
those arguments being specified, we can just backport that
patch (which applies cleanly), and not have to worry about it any
more.

pkgs/development/interpreters/spidermonkey/68.nix

···
     sha256 = "0azdinwqjfv2q37gqpxmfvzsk86pvsi6cjaq1310zs26gric5j1f";
   };
 
+  patches = [
+    # Backport a change from Firefox 75 that fixes finding the
+    # location of clang and libclang.
+    (fetchpatch {
+      url = "https://hg.mozilla.org/mozilla-central/raw-rev/ccd1356fc8f1d0bfa9d896e88d3cc924425623da";
+      sha256 = "005g3mfmal9nw32khrgyiv3221z7pazfhhm2qvgc8d48i2yzj3j0";
+    })
+  ];
+
   outputs = [ "out" "dev" ];
   setOutputFlags = false; # Configure script only understands --includedir
 
···
     "--enable-posix-nspr-emulation"
     "--with-system-zlib"
     "--with-system-icu"
-
-    "--with-libclang-path=${llvmPackages.libclang.lib}/lib"
-    "--with-clang-path=${llvmPackages.clang}/bin/clang"
 
     "--enable-shared-js"
     "--enable-readline"