系统环境：Ubuntu 18.04

sysY-Lexer测试方法：

//1.产生lex.yy.c
## lex lexer.l

//2.编译lex.yy.c
## gcc lex.yy.c

//3.运行测试程序 testing中是一段用于测试的sysY代码
## ./a.out <testing

Compared to C99 Standard, we cut off:

• wide char string literal and C++11 unicode string literals
• unnecessary keywords somehow like volatile, register, restrict and auto
• GCC extension like __attribute__ and builtin types and functions like SIMD __mm256d
• VLA, it's a C99 standard (removed in C++ standard)

• Basic numeric type support
• Linear control flow
• Pointers
• Variable shadowing
• Non-Linear control flow like for, if
• Arrays, struct, typedef
• Preprocessor macros
• Better front end diagnose output (may need hand written parser)