clang-7: error: linker command failed with exit code 1 for macOS Big Sur

This has been challenging, but here are the steps I used compile R packages from source on MacOS Big Sur:

1. Reinstall xcode command line tools

(don't believe Software Update if it says 'up to date' - he lies - brew doctor said my version was actually old)

sudo rm -rf /Library/Developer/CommandLineTools
sudo xcode-select --install

2. Install gcc & llvm via Homebrew (instructions for installing Homebrew) or, if you already have gcc/llvm installed, skip to the next step

# WARNING: This can take several hours
brew install gcc
brew install llvm

3. If you already have gcc & llvm installed via Homebrew:

brew cleanup
brew update
brew upgrade
brew reinstall gcc
brew reinstall llvm

4. Link some headers into /usr/local/include

sudo ln -s /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/* /usr/local/include/

# I believe you can safely ignore warnings like this:
#ln: /usr/local/include//tcl.h: File exists
#ln: /usr/local/include//tclDecls.h: File exists
#ln: /usr/local/include//tclPlatDecls.h: File exists
#ln: /usr/local/include//tclTomMath.h: File exists
#ln: /usr/local/include//tclTomMathDecls.h: File exists
#ln: /usr/local/include//tk.h: File exists
#ln: /usr/local/include//tkDecls.h: File exists
#ln: /usr/local/include//tkPlatDecls.h: File exists

5. Edit your ~/.R/Makevars file (if you don't have a file called Makevars in your ~/.R/ directory, create it) and include only these lines:

FLIBS=-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin19/10.2.0 -L/usr/local/gfortran/lib -lgfortran -lquadmath -lm
CXX1X=/usr/local/gfortran/bin/g++
CXX98=/usr/local/gfortran/bin/g++
CXX11=/usr/local/gfortran/bin/g++
CXX14=/usr/local/gfortran/bin/g++
CXX17=/usr/local/gfortran/bin/g++

LLVM_LOC = /usr/local/opt/llvm
CC=/usr/local/gfortran/bin/gcc -fopenmp
CXX=/usr/local/gfortran/bin/g++ -fopenmp
CFLAGS=-g -O3 -Wall -pedantic -std=gnu99 -mtune=native -pipe
CXXFLAGS=-g -O3 -Wall -pedantic -std=c++11 -mtune=native -pipe
LDFLAGS=-L/usr/local/opt/gettext/lib -L$(LLVM_LOC)/lib -Wl,-rpath,$(LLVM_LOC)/lib
CPPFLAGS=-I/usr/local/opt/gettext/include -I$(LLVM_LOC)/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include

6. Compile a package from source in R/Rstudio

# Compile the nlme package from source
install.packages("nlme", type = "source")

# To check whether openmp is enabled, compile data.table:
install.packages("data.table", type = "source")