询问一个编译原理的问题： implicit declaration of function 'yyerror' is invalid in C99

[EchoChan]

项目是根据<amp-youtube data-videoid="__-wUHG2rfM" layout="responsive" width="480" height="270"></amp-youtube> 做的，源码在https://github.com/jengelsma/yacc-tutorial
也就是两个文件，
一个https://github.com/jengelsma/yacc-tutorial/blob/master/calc.l
一个https://github.com/jengelsma/yacc-tutorial/blob/master/calc.y
利用命令生成y.tab.c和y.tab.h以及lex.yy.c后，使用gcc lex.yy.c y.tab.c -o calc命令，出现了如下错误
calc.l:11:8: warning: implicit declaration of function 'yyerror' is invalid in
C99 [-Wimplicit-function-declaration]
{ECHO; yyerror ("unexpected character");}
^
1 warning generated.
y.tab.c:1249:16: warning: implicit declaration of function 'yylex' is invalid in
C99 [-Wimplicit-function-declaration]
yychar = YYLEX;
^
y.tab.c:605:16: note: expanded from macro 'YYLEX'
# define YYLEX yylex ()
^
calc.y:46:5: warning: implicit declaration of function 'islower' is invalid in
C99 [-Wimplicit-function-declaration]
if(islower(token)) {
^
calc.y:48:12: warning: implicit declaration of function 'isupper' is invalid in
C99 [-Wimplicit-function-declaration]
} else if(isupper(token)) {
^
3 warnings generated.

Google了一圈，按照http://stackoverflow.com/questions/27220759/linker-error-yacc-on-mac
和http://stackoverflow.com/questions/23717039/generating-a-compiler-from-lex-and-yacc-grammar的说法都没解决。请问这怎么解决？是不是因为os x 内置的GCC编译器只能生成C99风格的代码而造成的？