I made a typo (forgetting the -D):

clang -Wno-unused-result -Wsign-compare -g -O0 -Wall -arch x86_64 -mmacosx-version-min=10.9 -Wno-nullability-completeness -Wno-expansion-to-defined -Wno-undef-prefix -isysroot /Applications/Xcode_12.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk -fPIC -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include/ncursesw -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include/uuid -Werror=unguarded-availability-new   -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal  -I. -I./Include -arch x86_64 -mmacosx-version-min=10.9 -Wno-nullability-completeness -Wno-expansion-to-defined -Wno-undef-prefix -isysroot /Applications/Xcode_12.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk -fPIC -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include/ncursesw -I/var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/tmpkfji88v7/tools/deps/include/uuid -Werror=unguarded-availability-new   -DMACOSX -DUSING_MALLOC_CLOSURE_DOT_C=1 -DHAVE_FFI_PREP_CIF_VAR=1 -DHAVE_FFI_PREP_CLOSURE_LOC=1 -DHAVE_FFI_CLOSURE_ALLOC=1 -DHAVE_DYLD_SHARED_CACHE_CONTAINS_PATH=1 -DPy_BUILD_CORE_BUILTIN  -I_ctypes/darwin -c ./Modules/_ctypes/callproc.c -o Modules/callproc.o