v0_8: A leading underscore is valid for macro substitutions.

For text macro substitutions the lexor did not allow leading underscores.
This patch fixes that omission. All other places that text macros may be
used appear to be correct.

ivlpp/lexor.lex

@@ -333,7 +333,7 @@ W [ \t\b\f]+
 <IFDEF_FALSE,IFDEF_TRUE,IFDEF_SUPR>`endif { ifdef_leave(); yy_pop_state(); }
 
   /* This pattern notices macros and arranges for them to be replaced. */
-`[a-zA-Z][a-zA-Z0-9_$]* { def_match(); }
+`[a-zA-Z_][a-zA-Z0-9_$]* { def_match(); }
 
   /* Any text that is not a directive just gets passed through to the
      output. Very easy. */