Defining PY_FORMAT_LONG_LONG had a side effect:

/usr/local/bin/clang -c -Wno-unused-result -Werror=declaration-after-statement -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -g3 -fsanitize=undefined -DPY_FORMAT_LONG_LONG=1 -g3 -fsanitize=undefined -DPY_FORMAT_LONG_LONG=1   -I. -IInclude -I./Include    -DPy_BUILD_CORE -o Objects/unicodeobject.o Objects/unicodeobject.c
Objects/unicodeobject.c:2325:15: warning: incompatible integer to pointer
      conversion initializing 'char *' with an expression of type 'int'
      [-Wint-conversion]
        char *f = PY_FORMAT_LONG_LONG;
              ^   ~~~~~~~~~~~~~~~~~~~
*****

export CC=/usr/local/bin/clang
export CXX=/usr/local/bin/clang++
export CFLAGS="-g3 -fsanitize=undefined -DPY_FORMAT_LONG_LONG=1"
export CXXFLAGS="-g3 -fsanitize=undefined -fno-sanitize=vptr -DPY_FORMAT_LONG_LONG=1"
...

./configure --disable-ipv6

make