How to compile a whole R source packages with Clang -Wno-nullability-completeness option in OSX 10.15

Based on several postings in stackoverflow, in OSX 10.15, I can suppress warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Wnullability-completeness] from conftest.c by clang -Wno-nullability-completeness -o conftest -g -O2 -I/usr/local/opt/gettext/include -I/usr/local/opt/readline/include -I/usr/local/include conftest.c or gcc -Wno-nullability-completeness conftest.c. But how can I implement this for compiling a whole R pacakge from the source ? I added -Wno-nullability-completeness in R Makevers like this :

CC=clang -Wno-nullability-completeness
CXX=clang++ -Wno-nullability-completeness
CFLAGS=-Wno-nullability-completeness -g -O3 -Wall -pedantic -std=gnu99 -mtune=native -pipe
CXXFLAGS=-Wno-nullability-completeness -g -O3 -Wall -pedantic -std=c++11 -mtune=native
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

but it did not work. Any suggestions or pointers will be seriously appreciated.

My system;

sw_vers
ProductName:    Mac OS X
ProductVersion: 10.15.2
BuildVersion:   19C57

gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/9.2.0_2/libexec/gcc/x86_64-apple-darwin19/9.2.0/lto-wrapper
Target: x86_64-apple-darwin19
Configured with: ../configure --build=x86_64-apple-darwin19 --prefix=/usr/local/Cellar/gcc/9.2.0_2 --libdir=/usr/local/Cellar/gcc/9.2.0_2/lib/gcc/9 --disable-nls --enable-checking=release --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-9 --with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl --with-system-zlib --with-pkgversion='Homebrew GCC 9.2.0_2' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues --disable-multilib --with-native-system-header-dir=/usr/include --with-sysroot=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk
Thread model: posix
gcc version 9.2.0 (Homebrew GCC 9.2.0_2)

clang -v
clang version 9.0.0 (tags/RELEASE_900/final)
Target: x86_64-apple-darwin19.2.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin

brew info R
R: stable 3.6.2 (bottled)
Software environment for statistical computing
https://www.r-project.org/
/usr/local/Cellar/R/3.6.2 (2,123 files, 58.5MB)
  Poured from bottle on 2019-12-14 at 09:37:12
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/r.rb
==> Dependencies
Build: pkg-config ✔
Required: gcc ✔, gettext ✔, jpeg ✔, libpng ✔, openblas ✔, pcre ✔, readline ✔, xz ✔

Flags:

CPATH=/usr/local/include
LDFLAGS=-L/usr/local/opt/llvm/lib
CPPFLAGS=-I/usr/local/opt/llvm/include
SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk