Add lexical analyzer

lexer.l

+%{
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "inter.h"
+#include "y.tab.h"
+void yyerror(char *);
+%}
+
+%%
+
+"while"	return WHILE;
+"if" return IF;
+"else" return ELSE;
+"print"	return PRINT;
+[a-zA-Z_][a-zA-Z0-9_]*	{strcpy(yylval.id,yytext); return ID;}
+0|[1-9][0-9]*	{yylval.int_value = atoi(yytext);   return INTEGER;}
+(0|[1-9][0-9]*)\.(0*|[0-9]*[1-9])   {yylval.real_value = atof(yytext);	return DOUBLE;}
+[-()<>=+*/;{}]	return *yytext;
+">="	return GE;
+"<="	return LE;
+"=="	return EQ;
+"!="	return NE;
+[ \t\n]+    ;
+.   yyerror("lexical error");
+
+%%
+
+int yywrap(void)
+{
+    return 1;
+}