Why not using abs() from stdlib?

Why does Arduino.h use its own abs() if stdlib has this already builtin. This causes me problems in build environments. I dont see any reason why one would prefer an Arduino definition. Can we get rid of it?
```
#define abs(x) ((x)>0?(x):-(x))
```

首先阅读 Arduino.h，并定位 issue 中所示的本地 abs(x) 宏。检查标准库和受影响的构建环境如何解析 abs，然后验证移除 Arduino 定义不会破坏现有的 Arduino API 构建或依赖它的用户。

由索引模型根据 Issue 内容生成。