clang error when installing on R4 macos 10.15.7

[bio-la]

Hi, thanks for the very interesting package, I wanted to try it but when installing with devtools i get a bunch of warnings at the compilation stage. the last one is this

clang++ -mmacosx-version-min=10.13 -std=gnu++11 -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -o Signac.so CommonUtil.o Hdf5Util.o MatrixUtil.o RcppExports.o SparseMatrixUtil.o Venice.o -L/Library/Frameworks/R.framework/Resources/lib -lRlapack -L/Library/Frameworks/R.framework/Resources/lib -lRblas -L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin18/8.2.0 -L/usr/local/gfortran/lib -lgfortran -lquadmath -lm /Users/fc7/Library/R/4.0/library/Rhdf5lib/lib/libhdf5_cpp.a /Users/fc7/Library/R/4.0/library/Rhdf5lib/lib/libhdf5.a /Users/fc7/Library/R/4.0/library/Rhdf5lib/lib/libsz.a -lz -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
ld: warning: directory not found for option '-L/usr/local/gfortran/lib/gcc/x86_64-apple-darwin18/8.2.0'
ld: warning: directory not found for option '-L/usr/local/gfortran/lib'
ld: library not found for -lgfortran
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [Signac.so] Error 1
ERROR: compilation failed for package ‘Signac’
* removing ‘/Users/fc7/Library/R/4.0/library/Signac’
Warning message:
In i.p(...) : 
installation of package ‘/var/folders/6f/r101x9pj4bj3grn96jv0725h000m5j/T//RtmpnOqaCZ/file16b7573eaa188/Signac_0.0.9.tar.gz’ had non-zero exit status 

my gfrortran and clang are:

$ gfortran --version
GNU Fortran (Homebrew GCC 10.2.0_4) 10.2.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

any help much appreciated!
thanks!