Make the error message of #define user-friendly

The error message for #define without double quotes surrounding the
string, e.g.,
  #define AAA BBB
  .end
is changed from
  test.frm Line 2 ==> Illegal valpoin for preprocessor variable AAA
to
  test.frm Line 2 ==> Illegal string for preprocessor variable AAA.
  Forgotten double quotes (") ?