Undefined reference to `_yyerror' when compiling with Flex and Bison

I'm trying to make a compiler for a mini Pascal-like language. I'm using Flex and Bison for this and I came up with this error.

My Flex file:

%{
#include "y.tab.h"
#include <stdlib.h>
#include <string.h>
#include <math.h>
void yyerror(char *);
%}

%%

[1-9][0-9]*     {
                yylval.i = atoi(yytext);
                return INT;
}

program     return PROGRAM;
or          return OR;
and         return AND;
not         return NOT;
if          return IF;
else        return ELSE ;
while       return WHILE;
"+"         return PLUS;
"-"         return MINUS;
"*"         return MUL;
"/"         return DIV;
"["         return LSB;
"]"         return RSB;
"{"         return LCB;
"}"         return RCB;
"("         return LEFTPAR;
")"         return RIGHTPAR;
":="        return ASSIGN;
"=="        return ISEQUAL;
"<"         return LTHAN;
">"         return GTHAN;
"<>"        return NOTEQUAL;
"<="        return LESSEQUAL;
">="        return GREATEREQUAL;

[a-zA-z][a-z0-9]*   {
                    yylval.s = (char*)malloc(strlen(yytext)*sizeof(char));
                    strcopy(yylval.s,yytext);
                    return ID;
}               

[ \t\n]+                            /* eat up whitespace */         

.                                   yyerror("Unknown Character");

%%
int yywrap(void) {
    return 1;
}

My Bison file:

%{
    #include <stdio.h>
    #include <string.h>
    int yylex(void);
    void yyerror(char *s);
%}

%union {
    int i;
    char *s;
};

%token <i> INTEGERNUM

%token PROGRAM;
%token OR;
%token AND;
%token NOT;
%token IF;
%token ELSE;
%token WHILE;
%token PLUS;
%token MINUS;
%token MUL;
%token DIV;
%token LSB;
%token RSB;
%token LCB;
%token RCB;
%token LEFTPAR;
%token RIGHTPAR;
%token ID;
%token INT;
%token ASSIGN;
%token ISEQUAL;
%token LTHAN;
%token GTHAN;
%token NOTEQUAL;
%token LESSEQUAL;
%token GREATEREQUAL;

%%

program: