从R 4.0.0开始，需要为R安装Rtools工具至PATH

从R 4.0.0（2020年4月发布）开始，R for Windows使用了一个全新的工具链捆绑包rtools40。

Starting with R 4.0.0 (released April 2020), R for Windows uses a brand new toolchain bundle called rtools40.

这个版本的Rtools将mingw-w64 gcc工具链升级到了8.3.0版本，并引入了基于msys2的新构建系统，使得R本身以及Windows上R包所需的系统库的构建和维护变得更加容易。

This version of Rtools upgrades the mingw-w64 gcc toolchains to version 8.3.0, and introduces a new build system based on msys2, which makes easier to build and maintain R itself as well as the system libraries needed by R packages on Windows. For more information about the latter, follow the links at the bottom of this document.

This documentation is about rtools40, the current version used for R 4.0.0 and newer. For information about previous versions of Rtools that can be used with R 3.6.3 or older, please visit this page.

To use rtools40, download the installer from CRAN:

On Windows 64-bit: rtools40-x86_64.exe (recommended: includes both i386 and x64 compilers)

On Windows 32-bit: rtools40-i686.exe (i386 compilers only)

安装教程参考：

1. R 安装Rtools       https://zhuanlan.zhihu.com/p/147573163

2. WINDOWS如何配置Path环境变量       https://jingyan.baidu.com/article/60ccbcebad4fe964cab197c7.html