mkcl: pull upstream fix for -fno-common toolchains

Without the change build on upstream gcc-10 fails as:

    ld: mkclgc.a(pthread_start.o):(.bss+0x0): multiple definition of
      `MK_GC_jmp_buf'; mkclgc.a(alloc.o):(.bss+0xc0): first defined here

pkgs/development/compilers/mkcl/default.nix

@@ -18,6 +18,13 @@ stdenv.mkDerivation rec {
       url = "https://github.com/jcbeaudoin/MKCL/commit/0777dd08254c88676f4f101117b10786b22111d6.patch";
       sha256 = "1dnr1jzha77nrxs22mclrcqyqvxxn6q1sfn35qjs77fi3jcinjsc";
     })
+
+    # Pull upstream fix for -fno-common toolchins like gcc-10
+    (fetchpatch {
+      name = "fno-common.patch";
+      url = "https://gitlab.common-lisp.net/mkcl/mkcl/-/commit/ef1981dbf4ceb1793cd6434e66e97b3db48b4ea0.patch";
+      sha256 = "00y6qanwvgb1r4haaqmvz7lbqa51l4wcnns1rwlfgvcvkpjc3dif";
+    })
   ];
 
   nativeBuildInputs = [ makeWrapper ];