It is executed using the gcc command:

% gcc -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -fprofile-generate -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Modules/gcmodule.o Modules/gcmodule.c
clang: warning: argument unused during compilation: '-fprofile-generate'
% type gcc
gcc is /usr/bin/gcc

I have no idea if you can even use something other than clang on Macs now. In any case, the default compiler should work to build Python out of the box, if necessary, by checking things during configure.