handling nested function for gcc

[feiwang3311]

1. flag to support inner functions or not (decided based on if we use gcc or tcc)
2. when supporting inner functions, generate them in function syntax int f(int x) { ... }
3. when not supporting inner functions, closure conversions
4. for cpp codegen compiled with gcc, it seems that we can either use cpp closure or inner functions (not sure if there are differences between them). How do we choose?

Outstanding Problem:
In handling closure codeGen of CPP, we replace the lambda_forward symbol with lambda.
that is to say: if x2 = lambda_forward(x3), then replace all codegen of x2 with x3.
This is current wrong because it generate invalid code for mutually recursive closures.