Install with devtools::install_github() fails to detect build tools

This is my first time trying to download a Github package, and I'm running into some trouble using RStudio v1.2.5033 on MacOS Big Sur v11.2.1.

Initially, when running

library(devtools)
devtools::install_github('xuyiqing/gsynth')

I was getting this error:

Warning in install.packages :
  installation of package ‘gsynth’ had non-zero exit status

Did some reading, and it looked like I needed to download Command Line Tools, so I downloaded XCode CLI, clang4, and gfortran. Now I am getting a pop-up message saying:

If I click "No", I get this error:

Error: Failed to install 'gsynth' from GitHub:
  Could not find tools necessary to compile a package
Call `pkgbuild::check_build_tools(debug = TRUE)` to diagnose the problem.

Running the above pkgbuild code in an R console, this is what I get:

Trying to compile a simple C file
Error: Could not find tools necessary to compile a package
Call `pkgbuild::check_build_tools(debug = TRUE)` to diagnose the problem.
rror: Could not find tools necessary to compile a package