False stack overflow of lexer.push(sc int)

Created by: xlab

https://github.com/cznic/cc/blob/6b9f540d7493aa7525835befbd47626add0ec4b7/lexer.go#L141-L144

func (l *lexer) push(sc int) {
    if l.scs >= 0 { // Stack overflow.
        panic("internal error")
    }

    l.scs = l.sc
    l.sc = sc
}

This can trigger with no reason, commented so could move on with a header not found, like /usr/local/include/SDL/SDL_stdinc.h:42:11: include file not found: stdarg.h (and 2 more errors) which is pretty easy to solve.