fbterm: fix cross compilation

pkgs/os-specific/linux/fbterm/default.nix

···
     ncurses
   ];
 
+  makeFlags = [
+    "AR:=$(AR)"
+  ];
+
   # preConfigure = ''
   #   sed -e '/ifdef SYS_signalfd/atypedef long long loff_t;' -i src/fbterm.cpp
   #   sed -e '/install-exec-hook:/,/^[^\t]/{d}; /.NOEXPORT/iinstall-exec-hook:\