dspam: add -fcommon workaround

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

    ld: .libs/hash_drv.o:/build/dspam-3.10.2/src/util.h:96: multiple definition of `verified_user';
      .libs/libdspam.o:/build/dspam-3.10.2/src/util.h:96: first defined here

pkgs/servers/mail/dspam/default.nix

@@ -53,6 +53,12 @@ in stdenv.mkDerivation rec {
   ] ++ lib.optional withMySQL "--with-mysql-includes=${mysql57.connector-c}/include/mysql"
     ++ lib.optional withPgSQL "--with-pgsql-libraries=${postgresql.lib}/lib";
 
+  # Workaround build failure on -fno-common toolchains like upstream
+  # gcc-10. Otherwise build fails as:
+  #   ld: .libs/hash_drv.o:/build/dspam-3.10.2/src/util.h:96: multiple definition of `verified_user';
+  #     .libs/libdspam.o:/build/dspam-3.10.2/src/util.h:96: first defined here
+  NIX_CFLAGS_COMPILE = "-fcommon";
+
   # Lots of things are hardwired to paths like sysconfdir. That's why we install with both "prefix" and "DESTDIR"
   # and fix directory structure manually after that.
   installFlags = [ "DESTDIR=$(out)" ];