gcl: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

ld: ./libgclp.a(user_match.o):(.bss+0x18): multiple definition of
`tf'; ./libpre_gcl.a(main.o):(.bss+0x326d90): first defined here

pkgs/development/compilers/gcl/default.nix

···
 
   hardeningDisable = [ "pic" "bindnow" ];
 
-  NIX_CFLAGS_COMPILE = "-fgnu89-inline";
+  # -fcommon: workaround build failure on -fno-common toolchains:
+  #   ld: ./libgclp.a(user_match.o):(.bss+0x18): multiple definition of
+  #     `tf'; ./libpre_gcl.a(main.o):(.bss+0x326d90): first defined here
+  NIX_CFLAGS_COMPILE = "-fgnu89-inline -fcommon";
 
   meta = with lib; {
     description = "GNU Common Lisp compiler working via GCC";