title: Error when trying to specify VHDL standard in GHDL
body: <p>New with GHDL. This is my VHDL code:</p>
  
  <pre><code>/*
  Name: test.vhd
  Description: Demonstrates basic design in VHDL code.
  Date: 04. August 2017 AD
  */
  
  --first part: libraries
  library IEEE;
  use IEEE.STD_LOGIC_1164.all;
  
  --second part: entity declaration
  entity myEntity is
      port(a, b, c: in STD_LOGIC; y: out STD_LOGIC);
  end;
  
  --third part: entity implementation
  architecture aom of myEntity is
  begin
      y &lt;= not b and (c or a and b);
  end;
  </code></pre>
  
  <p>Trying to compile this with <code>ghdl -s test.vhd</code> gives me this error: <code>test.vhd:1:1:error: block comment are not allowed before vhdl 2008</code>.</p>
  
  <p>So I try this: <code>ghdl --std=08 -s test.vhd</code> and I get this: <code>ghdl:error: unknown command '--std=08', try --help</code>.</p>