shallot: add -fcommon workaround

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

ld: src/shallot.o:(.bss+0x8): multiple definition of `lucky_thread'; src/error.o:(.bss+0x8): first defined here

pkgs/tools/misc/shallot/default.nix

···
     })
   ];
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: src/shallot.o:(.bss+0x8): multiple definition of `lucky_thread'; src/error.o:(.bss+0x8): first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   installPhase = ''
     mkdir -p $out/bin
     cp ./shallot $out/bin/