alan_2: add -fcommon workaround for gcc-10

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

    ld: smScSema.o:/build/source/compiler/smScSema.c:135:
        multiple definition of `charset'; options.o:/build/source/compiler/options.c:16: first defined here

Use -fcommon as a workeround until upstream updates the code.

pkgs/development/compilers/alan/2.nix

@@ -14,6 +14,10 @@ stdenv.mkDerivation rec {
 
   makefile = "Makefile.unix";
 
+  # Add a workarounf for -fno-common tollchains like upstream gcc-10.
+  # alan-3 is already fixed, but the backport is nontrivial.
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   installPhase = ''
     mkdir -p $out/bin $out/share/alan2
     cp compiler/alan $out/bin/alan2